An ASU (Application Software Update) contains an enhancement or a fix that
requires more than just a code change (may include new DD's, UDC's etc). I
don't believe it is possible to check the code changes until after it's
installed (someone may want to verify this).
It's a good idea to create an additional environment to test ESU's and
ASU's. This environment is in addition to your DEV, PY and PD environments.
There are a couple of good documents on the knowledge garden regarding ASU's
and ESU's. Search for ASU AND Install and go to the "Help You Install Main
Page" and follow the links from there for the ASU install guide.