Pretty simple really.
You've bought a ticket for a class (ticket class that is) which was not available at the time of purchase. TRIPS will let you do that as the class could become available and/or it will give you priority over most other standby status tickets. It is highly unlikely that there will be an ALT in a similar position as they will issue as many ALT's as there are commercial fares available.
Cat C's that are wait listed are treated as commercial standby, behind any regular customers of course, and ahead of CAT A 's, SRC's (wait listed 'firm' or stby).
The reason you can't check in online is because a CAT C ticket would normally physically 'load' you onto the flight. If the fare is not available, I don't believe that you can checkin online, has to be done at the airport.
The only time a standby Cat C makes economic sense is when there are a number of other standby passengers on Cat A's etc, you expect a number will be senior to you and you need to travel.
Good luck.
Last edited by myekppa; 16th Oct 2013 at 06:44.