Getting the Schema Details
All the schema details for all supported object types can be obtained by going to the REST schema page using the URL:
All the objects supported will come up in a list, each as a link that will take you to a page detailing which items of information will be returned when fetching that object.
In the case of users the schema shows what will come back when fetching the full user data using a URL like this:
If, however, you are getting a list of users or searching for one or more users with a query parameter like this:
Then you will get a subset of user properties on each user returned in the list. This is an optimization since users have such a large amount of data associated with them that returning all of those properties for each user on a potentially large list is very inefficient. Make sure your application accounts for this.