Within the pool community, the moniker of "pro" is based on the general rating by your peers.
If you enter a handicapped or pro-rated event, your entry fee many times is decided by your rating among your peers. For instance...a Pro pays $100...an A+ player pays $80....an advanced, or A player...pays $60...etc.
After a few events, the peer system is not easily fooled. many events will only allow certain "ratings" of player. The BCA league uses a classification system and basically a Grand Master is a pro level player, but they will also specify Pro.
Being classed as a Pro is highly detrimental to your earning potential at smaller tournaments, because quite simply, many will not allow Pros to enter.
A lot of events use the posted or previous years top 64 ranked players as the Pro list.
In my opinion, one of the determining factors is the quality and experience of ones mental game. I believe a lot of very good players may underestimate the role this plays in high caliber pool.
When you are competing against 63 pros, or 127 pros, everyone can run out at any moment, and may run a few. The matches are decided by 2-3 mistakes in 16+ games. The ability to be able to compete a a pro level is certainly not just how well you shoot.
There is quite a jump from an A+ player to a rated Pro player.