diff --git a/src/extension-analytics.js b/src/extension-analytics.js
index 35b1f39ab0d9..e2adcab3468e 100644
--- a/src/extension-analytics.js
+++ b/src/extension-analytics.js
@@ -122,7 +122,9 @@ class CustomEventReporter {
 
 
 /**
- * A builder class that enable extension elements to easily build a CustomEventReporter instance
+ * A builder class that enable extension elements to easily build and get a CustomEventReporter instance.
+ * Its constructor requires the parent AMP element.
+ * It provides two methods #track() and #build() to build the CustomEventReporter instance.
  */
 export class CustomEventReporterBuilder {
   /** @param {!AmpElement} parent */
@@ -139,6 +141,8 @@ export class CustomEventReporterBuilder {
   }
 
   /**
+   * The #track() method takes in a unique custom-event name, and the corresponding request url (or an array of request urls).
+   * One can call #track() multiple times with different eventType name (order doesn't matter) before #build() is called.
    * @param {string} eventType
    * @param {string|!Array<string>} request
    */
@@ -160,8 +164,9 @@ export class CustomEventReporterBuilder {
   }
 
   /**
-   * Call to build the CustomEventReporter instance.
-   * Should only be called after all eventType added.
+   * Call the #build() method to build and get the CustomEventReporter instance.
+   * One CustomEventReporterBuilder instance can only build one reporter,
+   * which means #build() should only be called once after all eventType are added.
    */
   build() {
     dev().assert(this.config_, 'CustomEventReporter already built');