Pretty much all reputable resources and companies for the most part are roughly inline with one another as to the working range and limits for LiFePO4 cells, with some slight variations. There are a couple unique outliers like Winston or Battleborn which have their own quirks due to chemistry or some other factor, but that is the exception to the rule.
Your 'manufacturer' is giving numbers that are extremely out of sync with the rest of the industry (specifically the 51.3V = 100% claim) assuming they are LiFePO4 cells in a typical "48V" 16S configuration. You should ask them for documentation, ideally from the
cell manufacturer that confirms the numbers they are telling you.
What
@fblevins1 tells you is correct, it all comes down to single cell voltages, everything else is just an extrapolation of those numbers by multiplying by the number of cells in series (x4 for "12V" (12.8V) x8 for "24V" (25.6V) x16 for "48V" (51.2V). Because individual cell specifications are what matters most here, the most authoritative document with respect to proper voltage limits (in my eyes) is the spec sheet from the
cell manufacturer. If you buy prebuilt batteries, it may not be possible to get this, but it is worth asking for, or at least asking the CS rep you are speaking with to confirm it themselves on the spec sheet or with an engineer.