pragma Implementation_Defined (local_NAME);
This pragma marks a previously declared entioty as implementation-defined. For an overloaded entity, applies to the most recent homonym.
The form with no arguments appears anywhere within a scope, most typically a package spec, and indicates that all entities that are defined within the package spec are Implementation_Defined.
This pragma is used within the GNAT runtime library to identify implementation-defined entities introduced in language-defined units, for the purpose of implementing the No_Implementation_Identifiers restriction.