These names are arbitrary (though they can do some signalling). Especially the "angel" and "seed" which are basically the same thing.
Basically an "investment round" is when you sell equity (and thus set a price) the rest is debt, both legally and practically. If you don't need to file form D you didn't sell anything!
My understanding is that the most important attribute of an A round is that it fixes a specific valuation on your company.
Lots of companies take numerous "seed rounds"; some are continuously funding during the seed stage. They're not out of "seed" until they take a VC round that imposes a valuation on them.
It's also popular to price a seed round (from 500k up, in my experience) using the "Series Seed" legal documents that are close to boilerplate now. As a founder turned investor, it seems that those docs could be streamlined into software easily. Most of the terms are standardized at this point, aside from board seats.
<$1m is considered "angel" round, and typically done on SAFEs/notes
$1-4M is considered "seed" round, and is SAFE/note or equity
$5M+ is considered "A" round, and is equity plus normally a board seat