Snap

U & V Snapping 

Turn on one of the U snapping modes to enable horizontal snapping to the grid when Space-MMB moving shells in UV space. The first mode will snap the left side of the shell bounding box to the grid. The second mode will snap the center of the shell to the grid. The third mode snaps the right side of the shell to the grid. The fourth mode will snap the shell relative to its current position. And the final mode snaps the shell bounding box to the grid. That is, it will snap which ever of the left or right edges is closest to the grid.

Turn on one of the V snapping modes to do the same for vertical snapping. Click on the arrows, or type a number in and tap Enter, to change the number of grid lines. Individual points and packing box corner handles, when moved with Ctrl-MMB, will also snap to the grid when they get close.

Points / Shells / Boxes 

Click on these boxes to turn off and on the snapping of UV points, shells and packing boxes.

Snap Points to Points 

Tick this option to also enable the snapping of UV points to other UV points, independently of the current grid snapping settings.

To temporarily turn off all current snap settings, stow/hide the Snap/Stack/Sort panel.

Snap Shells 

Move shells over the top of each other, then pick the shell (or shells) you want to snap, then click the Snap Shells button. If their boundary points are close enough, then the picked shells will snap to the same location and rotation as the other shells.

Boundary Pts 

Once the picked shells are aligned, click Boundary Pts to move and pin the close boundary points to the location of the boundary points of the other shells.

Dist 

Set the boundary points search radius; you can select between pixels or real world units. Click the Dist button to turn on circles indicating this distance on all picked shells in the UV view, plus also any close boundary points will be highlighted with red and green dots. If no points are highlighted, then the shells are not close enough (or the distance too small) and so they will not snap together.

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.

Sort

Select shells to sort them by the following criteria. If no shells are selected, then all visible unlocked shells are sorted.

UV 

Sort by the UV area of each shell, from largest to smallest.

3D 

Sort by the area of each shell in 3D space, from largest to smallest.

Num 

Sort by the number of polygons in each shell, form max to min.