Personally when it comes to communities I think in terms of Primary and secondary. I am not a fan of cross-posting but just because I find the addition to my profile horrid. I do like communities that also list posts as a tag which in some ways does pose a problem to communities. This is why:
Leo from what I know only shows posts that use their interface so the custom json attached to it saying which interface was used. So Leo in itself has positioned itself a Primary only category.

@Proofofbrainio though show posts and allow rewards on posts if they just have the tag, so proofofbrainio can be primary and secondary. I can't Post from there and list the Leofinance tag and still get Leo rewards. (might be wrong?)

So if I write for LeoFINANCE I agree that my post is primarily on topic. I agree with Hive account@rishi556 that then if it is about code etc that it somehow does relate to the finance spectrum. Leo and many are quite lenient on that and for the LEO community specifically it will take a single push to truly villainize those they consider milking etc.

I know Hive account@enforcer48 says code can be in stem but fucked if I am able to think of stem as anything other than ladeeda space and cutting frogs. However they are a Primary community, so if you post to stem then you agree that is 100% STEM to your knowledge obviously as one grows and feels comfortable in the community you maybe write a bit off topic and just like the category maybe.

Then you add your secondary tags for "discoverability" say Pal ;) , proofofbrain obviously, some actual topic tags such as python, data-analysis etc.

So yes many communities have positioned themselves as secondary and we need those, I do not like cross-posting if only because it looks shit so maybe that is an interface thing, I think you should be almost completely on topic within your Primary category/community.