Changes the existing polygon component selection to the border UVs on 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 Border the selection is converted to the border UVs on the UV shell associated with the previously selected faces.
To edit or otherwise modify the selected border UVs 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.