Okay, as a non poser user I'm confused. I know when I was setting up Diva I could set it to unimesh. What does Unimesh Poser actually change?
I know these things can be confusing. When PSS (Pixar Subdivision Surface) was first introduced (in Poser 10?), it couldn't be applied to split figures in Poser because it required unimesh geometry. The Morph Tool was the same thing - it was incompatible with split figures, and required unimesh geometry. For that purpose, SMS had introduced a way to convert split figures into unimesh geometry, and that's the command we find in the main menu to convert to "Unimesh Skinning".
This creates a version of the geometry that is unimesh, and it's only used for PSS. In some cases, you might also want to use Unimesh Skinning when using the Morph Tool, because it can spike out verts between split groups.
However, Poser is still internally using the split version for general operations, because it doesn't know how to handle single piece geometries. The new Unimesh Skinning is only used in PSS, optionally (but recommended) when using morph brushes, and when rendering. You can say this is a kind of chimera creature, which explains why PSS has such an impact on Poser performance.
With unimesh Poser, the figures will ALWAYS be a single piece geometry, so we no longer need to convert to "Unimesh Skinning". In other words, Poser would no longer split the figure groups into separate geometries when loaded. This has deep repercussions in how it works inside. As it is now:
* Groups can only exist as split geometries,
* Morphs are applied per specific groups, not the entire figure,
* Viewport selections only work if the groups are split,
* Skeleton binding can only be done per split geometry group.
etc....
All this will be gone in unimesh Poser. A lot will have to change on how it works internally. All the other Rooms will have to adapted to how the new core operates. One could say they will basically have to rewrite Poser. The deepest change ever done to it.