The shareholders' or operating agreement, the certificate of formation, the capitalization table, share certificates or membership records, board and member resolutions, and any management or employment agreements. Bank signatory authorities matter too. A slide showing percentages proves nothing; the signed instruments are what govern voting, appointments, and the power to bind the business.
Make the documents agree with each other
Inconsistencies between the capitalization table, the agreement, and the bank mandate are common and easy to fix early. Check that entity names, dates, and contribution amounts match across all of them, and that any side letter between the partners is disclosed rather than kept informal. Note documents needing translation or original signatures.
Keep each partner's transfer confirmations attached to their own contribution so the two funding trails never blur together. Add one document class that partners routinely leave until last and should produce first: each partner's own funding record, kept in a separate file with its own index. The lawful source and the path the money travelled must be shown for each contribution independently, and a single joint account into which both partners transferred before completion makes both stories harder rather than one easier.
Where a partner's funds arrive from a business sale, a loan, or a family arrangement, the supporting documents belong with that partner's file from the outset. Hypothetical example: two buyers of a web hosting business pool their contributions in one account three weeks before closing for convenience, and afterwards each has to demonstrate which portion of a single balance was theirs, which is a solvable problem that need never have existed had the transfers gone directly.