For a long time, PICKADD was a simple on/off variable.
When it’s on, or set to 1, every object you select is added to the current selection set. To remove objects from the set, you can hold down Shift and select them again.
When PICKADD is off, or set to 0, the most recently-selected objects – either by clicking directly or by windowing around them -- become the new selection set. Anything previously selected becomes unselected, unless you hold down Shift while clicking.
(An unexpected toggling of PICKADD is probably by far the biggest trigger of questions on the discussion groups…that and PICKFIRST.)
In AutoCAD LT 2011, a third setting was added to the PICKADD variable: it can now be set to 2 as well, and in fact is set to 2 by default.
The only thing this affects is the Select command. With PICKADD set to 2, objects selected with Select remain selected, so they can be used in another command right away. (Before 2011, you needed to use the “P” selection modifier to pick up objects from the Select command.)
It does *not* affect macros, though, which is why you still need the “P” in the macros I posted on Friday.