diff --git a/src/RolloutEvaluator.ts b/src/RolloutEvaluator.ts
index d39d0a4..63b9cb7 100644
--- a/src/RolloutEvaluator.ts
+++ b/src/RolloutEvaluator.ts
@@ -751,11 +751,12 @@ function getUserAttributeValueAsSemVer(attributeName: string, attributeValue: Us
 }
 
 function getUserAttributeValueAsNumber(attributeName: string, attributeValue: UserAttributeValue, condition: UserConditionUnion, key: string, logger: LoggerWrapper): number | string {
-  const number =
-    typeof attributeValue === "number" ? attributeValue :
-    typeof attributeValue === "string" ? parseFloatStrict(attributeValue.replace(",", ".")) :
-    NaN;
-  if (!isNaN(number)) {
+  if (typeof attributeValue === "number") {
+    return attributeValue;
+  }
+  let number: number;
+  if (typeof attributeValue === "string"
+    && (!isNaN(number = parseFloatStrict(attributeValue.replace(",", "."))) || attributeValue === "NaN")) {
     return number;
   }
   return handleInvalidUserAttribute(logger, condition, key, attributeName, `'${attributeValue}' is not a valid decimal number`);
@@ -765,11 +766,9 @@ function getUserAttributeValueAsUnixTimeSeconds(attributeName: string, attribute
   if (attributeValue instanceof Date) {
     return attributeValue.getTime() / 1000;
   }
-  const number =
-    typeof attributeValue === "number" ? attributeValue :
-    typeof attributeValue === "string" ? parseFloatStrict(attributeValue.replace(",", ".")) :
-    NaN;
-  if (!isNaN(number)) {
+  let number: number;
+  if (typeof attributeValue === "string"
+    && (!isNaN(number = parseFloatStrict(attributeValue.replace(",", "."))) || attributeValue === "NaN")) {
     return number;
   }
   return handleInvalidUserAttribute(logger, condition, key, attributeName, `'${attributeValue}' is not a valid Unix timestamp (number of seconds elapsed since Unix epoch)`);
diff --git a/src/User.ts b/src/User.ts
index 4dbae35..b337e0b 100644
--- a/src/User.ts
+++ b/src/User.ts
@@ -2,7 +2,12 @@ export type WellKnownUserObjectAttribute = "Identifier" | "Email" | "Country";
 
 export type UserAttributeValue = string | number | Date | ReadonlyArray<string>;
 
-/** User Object. Contains user attributes which are used for evaluating targeting rules and percentage options. */
+/**
+ * User Object. Contains user attributes which are used for evaluating targeting rules and percentage options.
+ * @remarks
+ * Please note that the `User` class is not designed to be used as a DTO (data transfer object).
+ * (Since the type of the `custom` property is polymorphic, it's not guaranteed that deserializing a serialized instance produces an instance with an identical or even valid data content.)
+ **/
 export class User {
   constructor(
     /** The unique identifier of the user or session (e.g. email address, primary key, session ID, etc.) */
@@ -11,7 +16,40 @@ export class User {
     public email?: string,
     /** Country of the user. */
     public country?: string,
-    /** Custom attributes of the user for advanced targeting rule definitions (e.g. user role, subscription type, etc.) */
+    /**
+     * Custom attributes of the user for advanced targeting rule definitions (e.g. user role, subscription type, etc.)
+     * @remarks
+     * The set of allowed attribute values depends on the comparison type of the condition which references the User Object attribute.
+     * `string` values are supported by all comparison types (in some cases they need to be provided in a specific format though).
+     * Some of the comparison types work with other types of values, as descibed below.
+     *
+     * Text-based comparisons (EQUALS, IS ONE OF, etc.)<br/>
+     * * accept `string` values,<br/>
+     * * all other values are automatically converted to string (a warning will be logged but evaluation will continue as normal).
+     *
+     * SemVer-based comparisons (IS ONE OF, &lt;, &gt;=, etc.)<br/>
+     * * accept `string` values containing a properly formatted, valid semver value,<br/>
+     * * all other values are considered invalid (a warning will be logged and the currently evaluated targeting rule will be skipped).
+     *
+     * Number-based comparisons (=, &lt;, &gt;=, etc.)<br/>
+     * * accept `number` values,<br/>
+     * * accept `string` values containing a properly formatted, valid `number` value,<br/>
+     * * all other values are considered invalid (a warning will be logged and the currently evaluated targeting rule will be skipped).
+     *
+     * Date time-based comparisons (BEFORE / AFTER)<br/>
+     * * accept `Date` values, which are automatically converted to a second-based Unix timestamp,<br/>
+     * * accept `number` values representing a second-based Unix timestamp,<br/>
+     * * accept `string` values containing a properly formatted, valid `number` value,<br/>
+     * * all other values are considered invalid (a warning will be logged and the currently evaluated targeting rule will be skipped).
+     *
+     * String array-based comparisons (ARRAY CONTAINS ANY OF / ARRAY NOT CONTAINS ANY OF)<br/>
+     * * accept arrays of `string`,<br/>
+     * * accept `string` values containing a valid JSON string which can be deserialized to an array of `string`,<br/>
+     * * all other values are considered invalid (a warning will be logged and the currently evaluated targeting rule will be skipped).
+     *
+     * In case a non-string attribute value needs to be converted to `string` during evaluation, it will always be done using the same format
+     * which is accepted by the comparisons.
+     **/
     public custom: { [key: string]: UserAttributeValue } = {}
   ) {
   }
diff --git a/test/ConfigV2EvaluationTests.ts b/test/ConfigV2EvaluationTests.ts
index 8f9a93c..331e6bf 100644
--- a/test/ConfigV2EvaluationTests.ts
+++ b/test/ConfigV2EvaluationTests.ts
@@ -232,7 +232,7 @@ describe("Setting evaluation (config v2)", () => {
       ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", 3, "<>4.2"],
       ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", 5, ">=5"],
       ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", Infinity, ">5"],
-      ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", NaN, "80%"],
+      ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", NaN, "<>4.2"],
       ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", "-Infinity", "<2.1"],
       ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", "-1", "<2.1"],
       ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", "2", "<2.1"],
@@ -241,7 +241,8 @@ describe("Setting evaluation (config v2)", () => {
       ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", "3", "<>4.2"],
       ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", "5", ">=5"],
       ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", "Infinity", ">5"],
-      ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", "NaN", "80%"],
+      ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", "NaN", "<>4.2"],
+      ["configcat-sdk-1/PKDVCLf-Hq-h-kCzMp-L7Q/FCWN-k1dV0iBf8QZrDgjdw", "numberWithPercentage", "12345", "Custom1", "NaNa", "80%"],
       // Date time-based comparisons
       ["configcat-sdk-1/JcPbCGl_1E-K9M-fJOyKyQ/OfQqcTjfFUGBwMKqtyEOrQ", "boolTrueIn202304", "12345", "Custom1", new Date("2023-03-31T23:59:59.9990000Z"), false],
       ["configcat-sdk-1/JcPbCGl_1E-K9M-fJOyKyQ/OfQqcTjfFUGBwMKqtyEOrQ", "boolTrueIn202304", "12345", "Custom1", new Date("2023-04-01T01:59:59.9990000+02:00"), false],