Short answer : Pharmaceutical Grade = licensed, approved, governed, regulated, tested for quality and purity, accurately dosed etc. Research Grade = unregulated, unknown quality/purity, unlicensed in most cases, possible under dosed etc.
However, there are grey areas in terms of research chems - some are licensed for certain things (just not providing medications to HUMANS <key word here is humans>) some are governed by certain state laws etc. It can get messy because some labs are legitimate labs and some are just shady companies using under dosed product to make some cash. Honestly in terms of what labs can and can't do there's a whole list of laws we could go back and forth on for days. In general, pharmaceuticals are guaranteed to be a certain dosage, contain certain ingredients, be approved to treat certain illnesses, have substantial research and clinical trials supporting their efficacy, and are manufactured under certain conditions. This is regulated by the government and strictly enforced. This is not the case with research chemicals.