They have different organisation structures, where Alma is a 501c Non-profit and Rocky is a backed by a B-Corp, so in my head I read one of those as a community linux ie. Debian-feeling, and the other aims to make money ie: Ubuntu-feeling. That's an extremely crude interpretation though, both need cash to support themselves, so both need to make money some how.

Day to day they should be by design, interchangeable. Rocky has a nicer logo... This kind of ugly exchange on reddit[0] always sort of soured my impression of Rocky though, didn't really pass the lebowski test.

0. https://www.reddit.com/r/linux/comments/qv6mg2/were_the_alma...