I just consider them all the same. They're all updates, some big, some small.
That's pretty much how it is. This is indeed technically "CU31" that's coming out tomorrow. "March Update" is just more digestible from a broad perspective, IMO, so we prob won't be leaning into "CUXX" as blog titles or the lead on social. "Content" in the "Content Update" is essentially anything that requires a client download (content is code, too, not just a map or cosmetic item) as opposed to the service-side updates we also perform regularly.
TLDR = call it whatever you want. There are multiple nomenclatures used internally, too. :)