Copying directly from Arne's blog post:
"Choosing the “DLC” option tells our matchmaking system that you want to be matched with players who also have the new maps above all else. This means you are essentially forcing yourself to ONLY play with players who also have the new maps installed. This can potentially mean long waits if there are no open slots in existing matches or if the system cannot create a new match where all players have the new maps installed.
If you have a party of friends who all have the new maps, have the “Matchmaking” option selected to “DLC” and THEN a new friend joins your party without the new maps, the system will recognize this and only place your party in matches playing the original on-disk maps to ensure that your party does not have to break up."
FYI. Have fun out there!