What does back letter mean?

back letter meaning in Law Dictionary

n deal that lists obligatios and rights that are not from the principal contract.

back letter meaning in Business Dictionary

Subsidiary or supplemental contract (contract) between a couple of parties drawn up to report their obligations and/or liberties that (for whatever reason) couldn't be within the initial or main agreement. A letter of indemnity, for example, is a back letter as it backs up a certain claim of a contracting party contrary to the various other.