In AD DS, the global catalog (GC) is a partial view of a forest's NCs, with these properties:
The GC view includes all domain NCs, the config NC, and the schema NC.
The GC view is partial. It includes all objects in the included NCs, but only those attributes defined as members of the partial attribute set in the schema NC (as specified in section 22.214.171.124). If the GC is an RODC, the attribute list is further restricted to those attributes not present in the filtered attribute set in the schema NC (as specified in section 126.96.36.199).
The GC view is read-only.
The GC has no state model impact outside the schema NC, which defines the forest's partial attribute set. The implementation of the GC (that is, actually providing the specified view to LDAP clients) does have impact, explained in section 188.8.131.52.9.
In AD LDS there is no support for the GC.