mc.gen~ uses the attribute to create multiple "voices" that are individual instances of the Gen DSP patches hosted by the object. It is a wrapped object, so it can use the functions available to any wrapped object in MC.
mcs.gen~, like other mcs.* object, allows the direct use of multichannel inputs and outputs for gen~ operations.
MC also introduces mc.gen, a variant of the event-based gen object. In addition to assigning a channel count to the object, it provides an additional outlet that informs Max of the voice number that generates any outgoing message.
MC Gen Guides
You can learn more about issues related to MC-based Gen patching by reviewing the following guides:
MC and Gen Example Patches