To my knowledge, it is common for she/they to mean "either she/her/hers or they/them/theirs is fine".

If a person tells you that they prefer one or the other, want you to use both in a rolling fashion, or some other specific arrangement like "he/them/his" then you should respect it, but in my experience that isn't normally expected just from saying "she/they" or "he/they".

It's also the case that some people use he/they or she/they because they want to go by she or he but aren't confident in their presentation and so they kind of give people an "out" by having they in there (not saying this is everyone, just that I've known a couple people like that). If you get that impression it might color your choice of what pronoun to pick, but its not really on you to suss that out

In my experience it depends on the person, and that person may change their mind at any time. For instance I know someone who prefers they if they look more enby or she if they look more fem (based on dress or makeup or even what they're doing at that time). I know someone who goes by she/they but they want me to call them they (because they know I can do it) but are okay with strangers or acquaintances calling them she. I know someone who is "anything but she" and also someone who is "anything but he." I know someone who is "not they" ... so she is good with she but he is also good with he, just not they (this human is an absolute agent of chaos). Most of these people have changed their feelings about their pronouns and/or how they feel about their relationship with gender at least once in the time I've known them, and usually they just inform me and I adjust.

So basically I just ask what is preferred if I can, most folks have a preference even if its slight. If they don't I use whatever feels natural. If they have their pronouns labeled somehow like an email or a nametag I use the first one they have listed. If I don't know them and there isn't clear context and nobody has told me and I haven't had the opportunity to ask, I use they until I am informed otherwise. I still get it wrong sometimes and apologize and try to get it right next time.

I'm curious to see what others think as well. Gender is a constant journey for a lot of people and I think it's neat. But basically to succinctly answer, I think it probably depends on the person. At least that's how it has been in all the spaces I'm in.

Some people are completely fine with all possible uses and combinations and you can just use whatever you feel like. Some are kinda fine with both, but actually do have a preference and view the second pronoun as a compromise that's ok-ish when they're around random cis people they don't want to give a ted talk about the complexities of their gender to. Some will want you to use both sets of pronouns alternatingly ("she asked me to come over, but i was busy, so i asked them if we could postpone"). Some (especially genderfluid) people will have a preference that varies over time and may have markers like a pronoun pin or clothing choice to signal which pronouns they go with at a given time, like she / her when in drag and he / him when in masc-coded casual clothes.

The tl;dr is that you really can't generalize what something like she / they means in practice and that you should just ask the person.

Edit: Also when people are out just recently, it's possible they're trying out which pronouns work best for them. In that case, it may be a good thing to help them get a feel for both by using both pronouns in different situations and, if you're close enough, asking them after a while if they're still using both sets of pronouns. A lot of people make the experience of introducing themselves with, say "they / he" and everybody just he / hims them 100% of the time, and in that case particularly, i'd ask how that person feels about this when some time has passed and there's a good opportunity. Such choices do not always work out as intended, and people can and do change their minds about the fine print when exploring their gender.

Yeah it always feels bad to me because I feel like I'm imposing my own thoughts onto the situation. Maybe I'll get used to it someday.