Given this part of the UC300 is based on Asterisk, this might be relevant:
http://www.venturevoip.com/vm.pdf
So, in a nutshell:
Busy greeting - The phone is actually in use (i.e. user is on the phone)
Unavailable greeting - The phone wasn't answered (ring timeout)
Temporary greeting - This overrides both of the above if it exists. This may be useful for 'I am on holiday' or 'I have left the company' type messages. If this message is deleted, then the existing greetings will be used. This allows you to have a temporary message without having to destroy/re-record your other two greetings.
Hope this is of use..
Steve