To be honest I´d say that´s up to you, because we all have our own limits and I don´t think that one limit applies to all of us. But there are definitely lines that you shouldn´t cross, for example when you´re a professional Dom/Domme you don´t have sex with your subs. If the temptation becomes too strong for you then it´s better that you stop with the session, you can talk about it with this sub and if the feelings are mutual then you can always start a relationship with this sub. It´s not impossible!
Sometimes subs try to seek out where the line is, for example sometimes there are subs that will ask for a handjob. Is that the same as sex? Not exactly but it´s still something that I wouldn´t do, if there are Doms/Dommes out there that would do that then that´s their business. As a Dom/Domme you could ask yourself if a handjob fits in your session, the same counts for subs that want to be fingered. I know that some Doms/Dommes decide when a sub is allowed to ejaculate but you don´t need to have your hands on for that!
A fetish that isn´t accepted by a lot of people is the golden shower, it´s not a surprise that subs are reluctant to talk about that when they´re interested in that. Once again it´s up to you if you´re interested in that, don´t do something you´re not comfortable with because that doesn´t make anyone happy. It´s better to refuse than to give a sub false hope. It´s smart to first talk with a sub before you start a session, especially if this is someone you don´t know much about. Hopefully this keeps confusion and misunderstandings out of the session.
There are also Doms/Dommes that don´t handle blood that well, this is nothing to be ashamed for and it´s better to be honest about this than to put yourself through something you don´t want and/or like! I seriously doubt a sub will think less of you because you don´t like blood, but this should also be something you should talk about before you start the session!
Unfortunately things can get much worse, for example some people like to include animals in a BDSM session. I don´t like that at all! I doubt that there are a lot of Doms/Dommes out there that want to do that. There is nothing wrong with making suggestions but sometimes it´s better to keep certain things to yourself, especially if you don´t know eachother that well. Of course things can get even creepier or weirder but I think you got the idea.
Don´t be afraid to say no, wether you´re a Dom/Domme or a sub. If you´re uncertain about something then don´t do it, it´s better to be safe than sorry! If you´re really curious about something then you can always do some research, there is nothing wrong with that. I would suggest that you stick with what you know and what you like, that usually leads to a good session and a lot of fun!