Offset is just that .. an Offset from the original position given ( <Location> ).
Relative - Relative to the Window you're in. Turn it off and reload your skin. And take a look. Then you'll see what that means.
If you ever wonder what an element is for, change it, reload the skin and look. Nothing more. Some are easy, some requires tinking a bit.
trust me ... I've just spent 3+ hours tinking with the freeking borders in EQ. It's a long pain processes ...
Good thing I don't have EQ to distract me ( this system can't play EQ, only load it enough to play with UI.
).