One is "Left/Right Side Surround" and one is "Left/Right Surround". I think the Left Side Surround is more like Dolby's configuration and just above ear level... The one with the rear and side surrounds more to the rear is the default. But whats the point in the subtle difference if our AVR's all read it the same (and from what I've tested my AVR's don't decode either 7.1 configuration different)? Is it really just for when the mixing engineer designates the side surrounds as 90º vs 105º~? I've been itching to figure this out but haven't been able to find this in a search or in the DTS MAS help PDF.
The DTS layouts require processors where the user can specify his speaker configuration. As far as I have seen, there are only a few processors with that functionality. So, it's not a feature anyone can use at this time.