In the Transfer Maps tool, a user-editable piece of geometry defining the search volume or threshold between target and source surfaces for texture maps (including normal maps, diffuse color maps, displacement maps, and lit and shaded maps). These texture maps are generated by the Transfer Maps tool ( Lighting/Shading > Transfer Maps).

In other words, a search envelope is a copy of the target mesh that you can scale up or down to include or exclude parts of your source meshes from the mapping process.