Let me answer generally first, then will share my personal situation.
Many people on the scene have very different boundaries. It's totally up to every individual and couple, and it's something that is always discussed in advance. (Or should be! If it's not, it would imply there are no hard boundaries, or someone is making a mistake if not discussing it, possibly if they are new to the scene!) it's a very common question to explicitly ask in advance, and unless it's totally obvious, during play....very often you hear permission sought multiple times during play .... 'can I touch you here....Would you like it if I....' or 'how would it feel to you both, if I...?' etc. if it's a first time play.
Some of the most common boundaries:
- some couples (relatively rare from my experience) prefer not to kiss others. Eg will do plenty 'harder sex' things, but associate kissing more with love and romance, so they withhold that just for their partners. For us, that would shift the sex towards a slightly unnatural, dehumanised transaction, which we wouldn't enjoy. But I understand why some couples do it.
Other couples may be described as 'soft swap' couples. That basically means they don't have penetrative sex with others, but will play with top halves, and may do oral. So in that scenario, the foursome may have full sex with their own partners, while touching etc the other people.
Full swap couples are happy to have full penetrative sex.
Then there are 'only play together' or 'same room players'. Or alternatively, 'play solo also', or 'separate room players'. Some couples explicitly prefer to play separately. For them it's about focusing on a unique experience with somebody else, rather than the group play aspect. They prefer not to be distracted by their own partner, and have a proper one to one encounter with the other person.... then often for those couples, reuniting/ telling each other all about it, is very intense and pleasurable for them. Generally, if couples play separately they are also ok about separate communications with people they may play with. Play-together couples are more likely to expect all communication to be including both of them. (Eg it's seen as a no-no, for someone to send chat messages just to one partner if it's a play-together couple, because that's not seen as respectful towards the relationship. It also gets very complicated to remember who knows what, and to be sure that everybody is genuinely happy with things.
You also have some couples, whose dynamic is more that one member of the couple (normally the man) just watches, while the woman has a full on experience with another man or woman. This can be that the voyeurism/exhibitionism is a form of kink linked to humiliation (called the cuckold dynamic). Or it can simply be that the man finds it beautiful and sexy to watch his gorgeous wife having pleasure, and has the confidence to step back and watch- either to provide a safe presence, and/or to keep it as a pleasurable experience between them too. . (This, as well as when the man is actively involved making it more of a threesome with the other man) is known more as the 'stag' dynamic)
Harder core stuff around Anal, or kinks, tend to be communicated on profiles too, with consent of course sought live in the room too.
The other thing to say, is it's very common for boundaries to start quite restrictive when a couple are first experimenting, then to expand if they feel they are enjoying it, they are becoming more relaxed/secure etc. so there are some over all definitions around the type of couple you are, but everybody on the scene will also know boundaries come down to what people want at the time with specific other people. Generally, I have found this lifestyle far more sensitive and sophisticated around boundaries and consent than the normal world.
So for us personally, we'd broadly be defined as a 'full swap play together' couple, and DH is definitely more s stag... zero 'cuckold' dynamic. We were 'soft swap for about the first six months of exploring'. There is nothing we have agreed to exclude from our play. I can also imagine we might move towards occasional solo plays soon, but not sure.
There are still some differences - both in feel and act- between the sex with others, EG there is definitely a much slower, intimate loving form of love making, versus sex, which I do with my DH sometimes, which I can't imagine feeling inclined to do with a play partner... but these aren't rules. At this point, We feel confident to allow ourselves to bring our full selves to the room and experience whatever we want to.