The Apple2 doesn't have sprites so the Apple2 mouse callbacks place a special character on the text screen to indicate the mouse position. In order to support the necessary character removing and redrawing the Apple2 mouse driver called the Apple2 mouse callbacks in an "unusual way". So far so (sort of) good.
However the upcoming Atari mouse driver aims to support both "sprite-type" mouse callbacks as well as "text-char-type" mouse callbacks. Therefore the interface between mouse drivers and callbacks needs to be extended to allow the mouse callbacks to hide their different types from the mouse driver.
The nature of this change can be seen best by looking at the Apple2 file modifications. The CBM drivers and callbacks (at least the current ones) don't benefit from this change.
