Snap

U & V Snapping 

Turn on U: Left, Center or Right to enable horizontal snapping to the grid when Space-MMB moving shells in UV space. Turn on V: Bottom, Center or Top to enable vertical snapping. Click on the arrows, or type a number in and hit Enter, to change the number of grid lines. Individual points, when moved with Ctrl-MMB, will also snap when they get close.

Points / Shells / Boxes 

Click on these boxes to enable snapping for UV points, shells and packing boxes.

Stack

The stacking tools can be used in two ways:

Multiple Shells 

Move an anchor shell away from other shells, then aim the mouse pointer at empty space and use the S hotkey to bring up the Stack menu, then LMB select that shell. You can then click Gather U or V to gather together picked shells that match the boundary of that selected shell. If no shells are picked, visible and unlocked shells will be gathered. Use the S hotkey to tag one point on the anchor shell, then click Match Point to copy that point location onto the gathered shells. You can then click on Stack to pile all the gathered shells on top of each other. The location of the tag point is used as an orientation reference, so it should be in the same spot on each of the shells (e.g. top left corner).

When you are happy with the alignment, click on Avg if you want to average all the boundary shapes together, or 1st to copy the shape of the first shell's boundary over to the other shells.

Once the boundaries have been reshaped, use the U or V buttons to arrange the shells in each of those directions, or you can leave them where they are to share texture space. The boundaries are pinned, so these shells can now be reflattened/optimized without worrying about the matching boundary shapes changing.

As a final step, click on Off to unset all the S tagged points.

Single Shells 

Use the S hotkey to tag one boundary point at the center-line of the shell you want to make symmetric, then click on Symm to rotate the shell so the center axis is vertical. Once you are happy with the alignment, click on Avg to average together the boundary points of the left and right halves, or use L or R to copy that side over to the other side. You can then use the V button if that shell needs to be symmetrical in the V direction (i.e. across the horizontal axis).

Note: This single shell stacking tool only makes the boundary shape symmetrical. If you want internal UVs to be perfectly mirrored, use the newer Symmetry Find tool.