The SUBCART is virtually backwards compatible and can also be used on advanced machines. I have a modified Atari 800 XL. Only the 1 MB or the Axxlon 4MB functions have to be inactive and you can continue to use the SUBCART as AVGCART. However, access to the different OS versions does not work on modified machines with some OS versions. If you have several machines, including unmodified ones, you can also use the SUBCART to its full extent, otherwise you can continue to use it as AVGCART.