When importing high resolution meshes that were Catmull-Clark subdivided in other applications like ZBrush, you can quickly rebuild the lower subdivision levels for the mesh in Mudbox.
Recreating subdivision levels (sometimes referred to as unsubdividing) can improve interactive performance compared to importing only a high resolution model as the base level in Mudbox. It also allows you to sculpt on the lower level of the mesh while preserving detail at higher levels.
When the process finishes, a Heads-Up Display message indicates how many lower subdivision levels were recreated. UVs on the mesh are preserved on the lower levels.