So there are two senses of type checking, the checking of what you get into that function, and whether you want that to be applied to changes to the frame variable once you get it.
(The practice of using function arguments destructively is part of Rebol's pursuit-of-"efficiency" history, although it makes debugging harder.)
Rebol has only had the first form (checking arguments) with no checking after that.
Now that we'll be having the ability to do the checking after, this could be a notational way of distingiushing the two, saying you actually want modifications to keep the type of the argument in that set. 