It should be simple but it's not so simple. A wellness HRT clinic may have a compounding pharmacy that abides by FDA regulations but in their day-to-day operations who the hell knows. I always interpreted it this way:
Human Grade: Taxed, fit, and ready for human consumption (USDA, FDA, etc approved)
Veterinary or Animal Grade: Taxed, fit, and ready for animal consumption but not fit or approved for human consumption by regulatory agencies. This doesn't mean much though because if some people get sick or die from poisoned beef it's recalled and then likely nothing. On the other hand, if "Sparky" or "Butch" die from a company's injectable B12, heads roll, and you're on the news portrayed as the devil!
Underground Lab: Well, LOL, speaks for itself. Not taxed, determined to be fit, or ready for ANY consumption, buyer beware, at your own risk, probably illegal, all that. Not approved for use anywhere.
As some others said though, each has it's place for certain things. Unless you want to look for human grade trenbolone enanthate.