Inside the realm areas, a lot of rules for physics and such are a bit different. But aside from various ‘passive’ things, there’s also what can be called ‘god magic’ (though there’s other terms in setting, often “Realm Psychoactive Energy” or “RPE” in a research context, because nobody likes using the word “Magic” in a scientific paper).
The basic system is that sentient beings have souls, and souls generate a sort of self-sustaining magic energy. There’s a LITTLE bit of extra that’s generated, but that normally dissipates. Magic does NOT work outside of the realms normally; harpies can’t fly, stormwings have to bear the full weight of their feathers, priests or avatars can’t use their own protections, and so on.
Gods are interested in collecting this energy; when discussing mortals, basically any soul is interchangeable in how much ‘extra’ there is. An individual’s soul can be ‘marked’ to a god. This allows the god to essentially harvest that little bit of excess power that their soul generates, banking it for their own use (or their clergy’s use), and also to sort of stake out a claim to the individual.
That’s the benefit to the god, but the benefit to the mortal is that marks (as a general rule) can’t be overwritten by another god. So once you get a mark, you’re probably keeping it, and nobody else can mark you (outside of Plot). To put it in game terms, you get a huge bonus on saves against certain magical effects from OTHER deities, but leave yourself totally open to the god that marked you.
On the other hand, if you DON’T have a mark and end up in realm, well, watch your back, because you’ve got no protection at all! So if you end up in realm and don’t have a mark, you can basically show up at a temple to a god of your choice (assuming you get there on your own) and volunteer and get one. That’s a “voluntary” mark.
“Forcemarking” is…well, basically what it sounds like. Divine press-ganging, essentially. You get waylaid along the way in realm, and have a mark placed on you involuntarily. Forcemarks often include higher levels of consequences, such as transformation into a (sentient) species aligned with the god, change of gender, age, language replacement, skill overwrite, or other things. Depends on a lot of factors depending on story needs.
A voluntary mark can be done in less than a minute; an involuntary mark takes a bit longer. Usually about 10, assuming you aren’t in a temple to the deity who’s collecting you. Forcemarks that involve more than just the attuning take an hour or more even inside of the temple, meaning that as a practical matter, the victim has to be incapacitated. Note that gods don’t have to do marks of any type themselves; they absolutely CAN, and actually can do it best of anyone, but their clergy are more than capable of doing it for them (and often the god has way more important things to be doing with their avatar anyway).
The reason for the penalties for Forcemarks depend but mostly fall into “Making you more useful to the god” or “making it harder for you to leave” because if you were to leave realm and stay out, you’re not doing them much good!