When adding a cylinder volume, what is the purpose of the property "decompose"?

Decompose splits cylinder into six four-edged faces

The Decompose option splits a cylinder into six four-edged faces. Caedium has multiple level undo and redo so don't be afraid to experiment.

Decompose splits cylinder into six four-edged faces

Thanks for your comment. Do these need to be reconnected before running a simulation?

Another question, are the velocities shown on a rotating volume, absolute or relative?

Decomposed cylinder is manifold volume

The decomposed cylinder is a valid manifold volume like all the other primitive volumes created in Caedium, so you don't need to stitch it into a volume.

Results velocity vectors are absolute. Field velocity vectors are interpolated to surfaces, so the surface values do not reflect your boundary conditions.