I probably am toooo old and slow, but I can not seem to find any benefits---(more like, ghastly time wasting complications)---to the new three Screen Size options in Carrara?

If there are, can someone help with a "Set of Rules" list which would make my life easier please--I feel  the 6.01 update is not  providing an answer at this point.

(I listed a set of dimension magnification ideas in another thread. These may be part of the rules to be enumerated.)

bwtr