I know real_sub group is used for grouping similar real_sub but once I create real_sub group where do I link to all real sub ?
did you read online help? It is quite obvious to do the link. You "simply" add on REAL_SUB level either on "Reference" or on "Inheritance" level the spec id of REAL_GRp.
Therefore: you do not link one REAL_GRP to "all" REAL_SUB but to a "sublist" of REAL_SUB; and this is done by user in daily business (aS only business knows which REAL_SUb is part of which "group" (REAL_GRP)