Hmm, I'm not subwife but I'll try to put my half-knowledge in anyway and hope that the more experienced folks on here will jump in...

From what I've heard there's different kind of collarings and they'll probably vary a lot between people, but generally it marks an official bond or commitment between a sub and his/her Dom/me. A lot of people seem to liken it to an engagement or marriage and it's often done in a ceremonial way. (Would be nice to hear some stories of people here who've been part of a collaring actually - maybe there is someone willing to share?)

To me personally, receiving a collar, being allowed to wear it for my Master, means that we both commit to the relationship...it marks a point where it becomes more than just casual fun or playtime, pretty much like going from dating to forming a relationship. That kind of step would probably be analogue to giving/receiving a training collar in BDSM terms. The next step, i. e. getting engaged or married, might be similar to what an official collaring would be about.

I tried to stick with the analogy of marriage here, but of course this comparison has a few flaws. Monogamy isn't really a rule for everybody in the lifestyle, so there might be subs with more than one Master or Masters with several subs - but to me it doesn't really change what the collaring is about. It's still a commitment between a Dom/me and a sub on whatever terms that have been negotiated.

Oh my, where are the people who REALLY know what they're talking about? lol