[scim] Clarification on Groups Schema membership attribute

[Brian Demers <[e-mail]>]

TL;DR - What are the official sub-attributes of "membership" items in the
Groups schema?

The Groups schema listed in section 8.7.1 lists the possible sub-attributes
for `members` to be: `value`, `$ref`, and `type`.
The example in 8.4, contains `value`, `$ref`, and `display`

Section 4.2,  "Group" Resource Schema, only makes reference to `id`,
`$ref`, and _hints_ at `type`

members
      A list of members of the Group.  While values MAY be added or
      removed, sub-attributes of members are "immutable".  The "value"
      sub-attribute contains the value of an "id" attribute of a SCIM
      resource, and the "$ref" sub-attribute must be the URI of a SCIM
      resource such as a "User", or a "Group".  The intention of the
      "Group" type is to allow the service provider to support nested
      groups.  Service providers MAY require clients to provide a
      non-empty value by setting the "required" attribute characteristic
      of a sub-attribute of the "members" attribute in the "Group"
      resource schema.


NOTE: Section 8.7.1 does state the following:
> Where permitted, individual values and schema MAY change

If this schema is not complete, is there an _official_ schema in JSON
that is?