Unavailable information in an assignment/copy expression - what happens?

When a copy expression references a variable that has not been set, what should happen.

The assign activity talks about the assign only succeeding if all of the copies succeed, otherwise the target variables are not affected. Does this mean that if one of the copies attempts to use an unset variable that it will fail, and therefore the assign will fail - then what happens? Does the next activity following the assign get executed?