"Set" sets the entered value to all selected widgets.
"Offset" adds the entered value to each selected widgets (use negative numbers to offset left/up).
"Spacing" makes the leftmost/topmost widget stay where it is and moves the rest so the spacing between the widgets equals the entered value.
You can also use formulas like "12+52".X:
(only visible if exactly one widget is selected)
For updates check my reddit post.