what does "research use only" actually mean

Term is everywhere and misunderstood in both directions.

What it means: the material isn't authorized for human or veterinary use. It's a classification about permitted use.

What it doesn't mean: that it's been assessed as safe for anything, that it's been assessed at all, or that it meets any particular purity or sterility standard. The label is about what a product is NOT cleared for. People sometimes read it as a quality tier and it isn't, it's the absence of a classification, not a lower one.

Worth being precise about because it's load bearing for how this whole space is framed, including this forum. Discussion here is about published research, not guidance for use.