In general scratching, hair pulling, gouging, anything that threatens eyes and chokes are forbidden, unless by prior agreement. In submission wrestling, smothers and scissors holds are generally allowed. Discuss the appropriate attire with your partner beforehand. In general, you should be able to tap to signal submission, and the hold should always then be released. Of course, you can agree to some other rules, but be sure you understand what they are.
Some people are into sleeper holds.
I am sometimes asked to recommend girls who will do sleepers. I will not make such recommendations. Sleepers work by pressure on the carotid artery, which shuts off the blood supply to the brain. There is too fine a line between unconsciousness and death here. We're not talking about some injury that you'll recover from.