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Assuming that we are dealing with a utility, the elements of the account number could reference some components of the grid.
"Sub" could indicate a power or switching sub-station. In this context, the French equivalent would be "poste" as in "poste de transformation".
"Major" would be the organizational unit above the sub. I'm suggesting "groupe" which is generic enough to suit different industries. Another suggestion is "division".
If we are unsure of the context, then I would make it more general and use abbreviations. Something like:
No. prim. as in numéro primaire
No. second. as in numéro secondaire.