I had this question a long time now.
Who can we call a HUSNG "reg"?

One who:
-is a steady winner every month and in the leaderboards?
-grinds a lot?
-grinds a lot and wins a lot?
-has roi adj over 5%? and over a certain amount of games?
-open sits lobby and defends them regardless of their roi adj?
-has excellent GTO knowledge but loses to fish because he cannot adapt?
-knows nothing about GTO but crushes fish all day long, at a certain stake?
-plays really weird and spazzy, but still manages to have decent results every month?

Who of the following people is a "reg"?

Person A defends his lobbies (belongs in the division), is a steady winner and grinds a lot.

Person B defends his lobbies but rarely plays because he focuses on great game play and doesn't mind low game volume.

Person C defends his lobbies but is being crushed by fish because he is not up to the latest population tendencies or refuses to study. His roi adj and winnings are absolutely horrible but he belongs in the stake division.

Person D does not belong in the divisions but bumhunts certain "division regs" that he can crush.

Person E only bumhunts specific fish that opensit lobbies. He has over 8% roi adj and huge winnings versus them.

Person F is open sitting lobbies without being in the division, accepts every match, is doing really weird gameplays, donk shoves flops etc (lol) and has resonable winnings. Doesn't respect divisions.

Person G only sits one or two regs he can crush and is not interested in joining any divisions.

Player H is a random guy that does really “stupid” plays that actually work on opponents so he steadily wins. Doesnt select games.

Cheers!

as REG should be considered any player that beats the games and is playing average more that 1500 games or something per month.

And how is it possible to win other regs but getting crushed by recreational players?

You are right, in the long term it is not possible. But in the short term it happens that a reg cannot properly adapt to some wild recreational players in certain situations, for example difficulty in adjusting to rec's mixed flop donking (and barelling) strategy with various sizes. While playing versus a reg, the reg opponent will not be as wild with flop donking.

Imo someone who plays regularly and is not sat by other good regularly playing players at his limit.
I guess even if some people consider this player weak enough to sit, this reg will be called fishreg or weak reg but still reg.