add event info

neps/nep-504.md

@@ -142,14 +142,24 @@ Whenever a metadata is updated, an event should be emitted. We propose to follow
 - Events must include fields necessary to identify subject matters related to use case.
 - When possible, events should contain aggregated data, with respect to the standard function related to the event.
 
+The Contract and Class Metadata are stored and managed by the Issuer contract. This NEP covers only the Issuer contract. `TokenMetadata` is stored in the SBT registry, consequently Token Metadata updates and events are defined in NEP-393.
+
 ```typescript
 type Nep504Event {
   standard: "nep504";
   version: "1.0.0";
-  event: "token" | "class" | "issuer";
-  data: Token | Class | Issuer ;
+  event: "class" | "contract";
+  data: Class | Contract ;
+}
+
+/// An event emitted by an SBT Issuer when Class Metadata are updated.
+type Class {
+  classes: ClassId[];  // list of class IDs whose metadata has been updated
 }
 
+/// An event emitted by an SBT Issuer when Contract Metadata is updated.
+/// Note: it's an empty object
+type Class { }
 ```
 
 ## Reference Implementation