To carry on from what Karley said, I believe that Dominant is how someone considers themselves overall, while Master or Mentor implies some kind of bond.

To draw from my own experience, I consider myself a Dominant overall, this is my personality, my inclination. I would only require people who may sub for me to refer to me as Master/Mistress or any other related title. Such title is kept for people I am considering as submissive, or actually own as submissive.

I also had the case of people using such titles with me as a polite gesture even when it wasn't such a situation, but simply because as submissives they did that with the various Dominants around. I have no problem with that.

Now onto the "Mentor" thing, I believe that it concerns a situation, a "relationship" where it remains "purely" on the educational side, where a bond such as Master/slave isn't the ultimate goal, but to help a sub find themselves, or get some training shall they wish so.

In both Master and Mentor case, there is a huge need of trust and honesty required, as I believe even a Mentor/"student" bond has to be truthful, and shall not be taken lightly, but it's the relationship "goals" and "settings" that are different, though I think it's possible a Mentor becomes a Master to someone shall the relationship develop in a certain way.