A UserProperty is an attribute that describes the app-user. By supplying UserProperties, you can later analyze different behaviors of various segments of your user base. You may supply up to 25 unique UserProperties per app, and you can use the name and value of your choosing for each one. UserProperty names can be up to 24 characters long, may only contain alphanumeric characters and underscores ("_"), and must start with an alphabetic character. UserProperty values can be up to 36 characters long. The "firebase_" prefix is reserved and should not be used.
The following user property names are reserved and cannot be used:
|String||SIGN_UP_METHOD||The method used to sign in.|