"Formal potential" is just translation from Polish - I think the same name is used in English, although I am not 100% sure.
Formal potential is the potential observed in specific circumstances (solution).
For example for permanganate solution, potential depends on the pH:
E = E0 + RT/5F ln ([MnO4-][H+]8/[Mn+2])
If you know that pH will be constant, you can write the same equation as
E = E0' + RT/5F ln ([MnO4-]/[Mn+2])
moving pH into E0' - now it is a formal potential of the solution, equal
E0' = E0 + 8RT/5F ln ([H+])
That's just a mathematical trick, but very convenient one.
So, in your case, potential was given not for standard solution (where activities of all ions are 1) but for most likely to be used solution (neutral one) - that's application of the same approach.