oh I hate this question... I used to ask it.. but I think that you can't guarantee the strength of the medicinal value in a non "species" plant... sometimes the hybridization breeds it out. I know that viola odora and viola tricolor are the breeds listed in my favorite herbal as the useful ones.
I finally learned that officinalis means medicinal/edible, now if all medicinal herbs could be labeled like that we'd have it a lot easier.

I just eat my violas, flower and leaf. I don't worry about the strength of any medicinal property.

Viola labradorica is the Labrador Violet, a Canadian native whose range extends to New England, Wisc. & Mich.
Viola odorata and V. tricolor are both European natives which have naturalized in the US. None of the above are hybrids.
When properly written, X in the latin name designates that the plant is a hybrid. It should be noted that many plants including violets hybridize freely in nature.
Early 19th century botanists assigned the species name officialis to many plants both woody and herbaceous, its meaning is simply "common", having no herbal designation.

I believe vulgaris means "common" and officinalis means "the official species used for pharmacy".

Herbalbetty is correct.

I have recently read that all violas have the same medicinal value.

Sepia officinalis has no medicinal value, officinalis just means official, not what specifically it is officially used for.