diff --git a/tools/gulp/task_helpers.ts b/tools/gulp/task_helpers.ts
index b4af11325479..68f9f8e24b79 100644
--- a/tools/gulp/task_helpers.ts
+++ b/tools/gulp/task_helpers.ts
@@ -208,7 +208,10 @@ export function isTravisPushBuild() {
   return process.env['TRAVIS_PULL_REQUEST'] === 'false';
 }
 
-/** Open Google Cloud Storage for screenshots */
+/**
+ * Open Google Cloud Storage for screenshots.
+ * The files uploaded to google cloud are also available to firebase storage.
+ */
 export function openScreenshotsBucket() {
   let gcs = gcloud.storage({
     projectId: 'material2-screenshots',
diff --git a/tools/gulp/tasks/screenshots.ts b/tools/gulp/tasks/screenshots.ts
index 539a70493d7a..05143c823d8e 100644
--- a/tools/gulp/tasks/screenshots.ts
+++ b/tools/gulp/tasks/screenshots.ts
@@ -139,7 +139,10 @@ function diffScreenshot(filename: string, database: admin.database.Database,
   }
 }
 
-/** Upload a list of filenames to firebase database as gold. */
+/**
+ * Upload a list of filenames to firebase database as gold.
+ * This is necessary for control panel since google-cloud is not available to client side.
+ */
 function setScreenFilenames(database: admin.database.Database,
                             prNumber?: string) {
   let filenames: string[] = getLocalScreenshotFiles(SCREENSHOT_DIR);