Most/all of the Maps Platform APIs have a per user quota.  How is a user defined and measured?

The quota per user is the maximum amount of requests that a single user can make to the API in a given time period.  The per user quota is listed on the Quotas page as Requests per 100 seconds per user.  For example, for Maps JavaScript API the quota per user is 100 per 100 seconds which equates to one Map load per user per second.  

JavaScript API special considerations

The per user quota as listed in the Quotas page, is only applicable to Web Service requests.  In the JavaScript API, user session counts and quota enforcement are handled automatically by the API.  The JavaScript API has implicit quotas that are not published.

When using the JavaScript services to call the Maps Platform APIs via JavaScript, requests are capped by a mechanism internal to the API, unrelated to per user quota.  For example, Geocoding API with JavaScript API you are using the Geocoding Service and the API is being used "client-side".  The API will stop responding to requests after hitting a limit.  This limit is not the same as the per user quota and is not published nor documented well.  Client side APIs were not designed to execute a high number of simultaneous requests and are subject to a limit sufficient for a human user.

References

https://cloud.google.com/apis/docs/capping-api-usage#limiting_requests_per_second_per_user