L T W H   -> (L)eftEdge, (T)opEdge, (W)idth, (H)eight

These 4 values are the sizes you declare for windows, gadgets & ASL 
requesters. Apart from normal numbers, you can also use the following
numbers, which have special meaning :

L = LeftEdge  - if L is a negative value :
                - If used with a window or requester, then these will be
                  centered horizontally on the screen.
                - If used with a gadget, then the gadget will be centered
                  horizontally in it's window.

T = TopEdge   - if T is a negative value :
                - If used with a window or requester, then these will be
                  centered vertically on the screen.
                - If used with a gadget, then the gadget will be centered
                  vertically in it's window.

NOTE : The Left & Top sizes you declare will be offsets INSIDE the window.
Do not account for the window border, it will be added automatically.

W = Width     - if W is less than or equal to 0, then :
                - If used with a window or requester, their width will
                  be the SCREEN WIDTH minus this number
                  i.e. -1 will make a window 1 pixel smaller than the
               screen size, and if 0 it will be screen sized.
                - If used with a gadget, the width of the gadget will
                  be the width of it's window minus this number.

H = Height    - if H is less than or equal to 0, then :
                - If used with a window or requester, their height will
                  be the SCREEN Height minus this number
                - If used with a gadget, the height of the gadget will
                  be the height of it's window minus this number.

The only case where the above does not apply, is to the WinSmall
command - look at it to see the meanings.