sometimes they only swallow because they don't want the guy to think she finds it disgusting or some just like the taste of it. when they don't swallow it maybe only because they're afraid of STD-s or they don't like the taste of it.
They are talking about the result of them providing oral sex on a guy and him having come in her mouth. They either spit it out or swallow it.
It doesn't mean anything about the guy or the gal if they do one or the other so don't attach any meaning to it. There are also women who won't let it get in their mouth.