This feature would do exactly what it says - it would allow the composer to insert measure clips around the staff that signify and optional or alternate way to play a measure.

I don't know how to explain it, but the only real world application that comes to my mind when I think of this is "ASLAP" by John Cage. A picture of what I am talking about can be found at this link: http://artsaha.org/wp-content/uploads/2007/08/aslsp5.JPG

This has come up as an option that I, as a percussion composer, would find very useful for putting rough drafts into the computer. This obviously had implications in melodic composing as well.