The Tape In and Digital/Firewire Return on a StudioLive mixer will only be assignable in the Phones/Monitor section of your mixer or directly assignable to the Main output of a StudioLive AI Console or StudioLive Classic mixer (including the discontinued 16.4.2 and 24.4.2 mixers).
The StudioLive RM Series rack mountable mixers have an assignable Subgroup function, making them different in design than the StudioLive AI Console mixers and therefore give a more flexible functionality to your setup.
NOTE: Over AVB functional networks through AVB Stagebox Mode, the Digital return on AI console mixers will listen to the Tape In and other digital sources assigned over Firewire OR network with firmware versions greater than 7045 on AI Consoles and 7049 on RM mixers. These assignable functions are done in the RM mixer only through UC AI 1.5 and greater, and not on the console. Refer to the AVB Knowledge base area for more information on current AVB functionality.