Use these options to set what happens when you select.
Select Collectively to normalize the UVs for all selected faces collectively. That means the texture coordinates for all selected faces are “collectively” fit to the 0 to 1 texture space. This is the default.
Select Each face separately to normalize the UVs for each selected face separately. That means the texture coordinates for each selected face are fit to a boundary of 0 to 1.
Turn this option on to scale the UVs uniformly along U and V.
Turn this option off (the default setting) to stretch the texture to fit by scaling U and V non-uniformly.
When you normalize texture coordinates, you scale the UVs of the selected faces. If Preserve Aspect Ratio is on, the scaling is guaranteed to be uniform on both the U and V axes. If turned off, the scaling will be different for the U and V axes.
When on, scales the selected components or UV shell(s) within the closest UV tile.