I found a discussion thread about this in the Sims 3 Forums, but the original article was from SimTimes. Here is the article exert (loosely translated from German):
Did you create your Sims, you can in the relationship now, the options “Select Fixed friends Festivals friend” and “fiance / fiancee.
The just of the article is that in CAS you can make your sims go steady, be best friends or fiances. This should be a great feature to use.