[trompetenaccoun]

Anything can be marketed as "garum". The name isn't protected, all it takes is so naming whatever product you have. If our definition is some sort of fish sauce with various unspecified ingredients, then any Asian or other fish sauce is garum.

[jillesvangurp]

Exactly. In Roman times, they probably also had endless variations of what they called garum. There wasn't really any food authority. They would likely have used what would have been available to them locally. Mostly that would have been what they grew locally + what they fished out of the Mediterranean + what they could get their hands on via trade.

The recipe probably wasn't set down in stone. Which was of course one of the ways to publish information at the time. But mostly people just copied what other people were doing at the time. There weren't many recipe books/scrolls or literate cooks even. And it sounds like the process of making garum basically involved a lot of rotting fish guts so that doesn't strike me as something as a likely career choice for somebody with an actual education at the time.

So, basically the recipe would have gone something like: take whatever off cuts of fish, fish guts, etc. you have, add salt, and let it do its thing for some time. You get different results based on what fish you use, whatever else you might toss in (herbs, spices, etc), how long you let things ferment, etc. Probably all those variants would have been considered garum. Probably a lot of trial and error involved to get that right. That's also more or less how Asian fish sauces work.

[trompetenaccoun]

Agreed but to use a modern analogy, while there are thousands of variants of kimchi, none of them taste like sauerkraut.

Lots of seemingly minor details can make a big difference in cooking. From the Asian sauces for example I know it even makes a difference what fish species is used. It's possible that there are fish sauces out on the market that are similar or even essentially the same as variants that were used in Ancient Rome but we don't know this.