1. Center And Fit Shells

Center and Rescale the selection to fit the 0-1 space. If there is no selection, all the Shells are affected.
Fit: Fits the Shells to the 0-1 space.
Keep Ratio: Keeps the proportions of the selection. If unchecked, the selection will be rescaled not proportionally to fit the 0-1 space.
Padding: The space from the texture borders.
Shift + Click: Center and Fit by Shell.

2. Normalize Shells

Rescale the selected Shells to have the same pixel density. If there is no selection, all the Shells are selected.
There are three options, Average the pixel density among the selected Shells, use the minimum pixel density or use the maximum.
Fix UwVw: Fixes the W vertices position. If the vertices are not all at the same W position, wrong areas are measured.

3. Scale Shells

Scale the current selection around the selection pivot point by the specified factor. Additionally you can specify in which axis you want to scale the Shells, X horizontally and Y vertically.

Ctrl + Click: Scale the Shells around the selection center.
Shift + Click: Scale the Shells individually.
Alt + Click: Invert Scale factor

4. Match Shells Scale

Scale the selected Shells to match either their Widths or their Heights.