Changes the existing polygon component selection to the UV shell that is associated with the polygon components originally selected.
For example, if you have a polygon mesh with several faces selected and then choose Select > Convert Selection > To UV Shell, the selection is converted to the UV shell associated with the previously selected faces.
To edit or otherwise modify the selected UV shell you must use the UV Texture Editor. (Edit UVs > UV Texture Editor)
Many of the convert selection features are available from the context sensitive polygon marking menu when you press 
+ 
whenever a polygon component is selected.