Page 39 of Firebird 3.0 Release Notes (23 July 2013 - Document v.0300-06 - for Firebird 3.0 Alpha 1) mentions:
Currently, RDB$IDENTITY_TYPE always stores the value 0 (GENERATED BY DEFAULT) for identity
columns and NULL for non-identity columns. In the future this column will be able to store the value 1
(GENERATED ALWAYS) when that type of identity column is supported by Firebird.
It appears to be the otherway around (see also rdb$types): value 1 represents GENERATED BY DEFAULT and is currently always the case, and value 0 represents GENERATED ALWAYS (for future use).