This lets the server admin set the groups someone in a specified group can set other users to using PermissionsBukkit.
Example:
I have three admin groups:
DIV
BTN
SS
Then I have non-admin groups:
Visitor
SM
I want to allow DIV to be able to change anyone's group so I give that group the PermissionsBukkit setgroup permissions. For BTN I want them to only set people to Visitor, SM, or SS from those three same groups, they should NOT be able to touch DIV members or other BTN members. SS should be able to set peoples groups to Visitor from default, and that is it.