I often get asked 'Which is the real Comfrey'? Here's an answer...
'True (or Common) Comfrey' (Symphytum officinale) is a common herb in Europe. The other common variety is 'Russian Comfrey' (Symphytum x uplandicum) in many cases. This is, in turn, a hybrid of two varieties - that's what the 'x' in its taxonomic name means.
'True Comfrey' will seed prolifically, while the 'Russian Comfrey' we often buy as Comfrey is a sterile hybrid but is far more vigorous and will only grow from root propagation.
Our 'True' Comfrey has white to pink flowers, while Russian Comfrey has blue to purple flowers. Of course, 'True Comfrey' has a red/purple variety, just to make it all the more confusing.
If you want to know which variety you have, wait until it flowers. That's the easiest way. If you have a chance to hold a flower of True Comfrey next to a flower of Russian Comfrey, the True Comfrey will have the longer lobes on the sepals (they're the little green petals under the true petals in a flower). Otherwise, True Comfrey has white to pink flowers and more of a stem at the bottom of the leaves.
But is it worth all the fuss? Probably not.
Purists will only accept S .officinale for medicinal use and S. x uplandicum for feeding animals and providing biomass, but thats probably because S.x uplandicum is far more vigorous and grows faster.
In reality, there are more than 30 species in Europe that are all called 'Comfrey' and have been used interchangably over many, many years, even before they were introduced to the US in the 1700's.
Whichever you have, there was an Herbalist named Gerard who recommended the juice of comfrey root in ale for 'the paine in the back gotten by wrestling, or overuse of women.'
Use what you've got, I say!