The Fit UV command scales the currently-selected UV data to fill the normalized 0 to 1 UV space. This is a very useful command for optimizing UVs. There are several options for this command that controls how it fits the target UV data.
• Fit - controls how the UV data is handled: Entire UVs treats all the UV data as a single grouped element, Per UV polygon treats every polygon individually (resulting in a Barycentric UV layout), and per UV island treats connected islands of polygons as individual groups.
• Keep Proportion - when this option is active, the UV data scales proportionally so that the resulting UV data has the same ratio of height to width. The data scales until one of the maximum UV sizes is fit on one or both axes, but the data is not deformed to accomplish this.