UV Packing
In this UV Manager menu, you will find various settings for optimally re-arranging UV Islands.
But isn't this what the Optimal Mapping settings do as well? Yes, but this is a part of what Optimal Mapping does. As you know, Optimal Mapping takes place in multiple steps:
Unwrap UV mesh
Relax UV mesh
Pack UV mesh
The settings in this menu restrict the process to three steps: the optimal arrangement of the existing UV Islands without creating new ones (exception to the rule, see Rasterized). For the most part, this ensures an efficient use of the UV space so as not to waste any texture surface.
Use the Polygon Selection to restrict the packing to specific regions.