Users in Plone have a set of properties defined by a default set of fields such as
portrait, and other fields.
These properties define the site user's profile and the user itself via the Plone UI, or the site managers can add them in a variety of ways including PAS plugins.
These fields are dynamic and customizable by integrators so they do not adhere to a fixed schema interface. This dynamic schema is exposed by this endpoint in order to build the user's profile form.