A continuous trail: how the funds were lawfully generated, every account and transfer between then and now, the loan terms if any, and the payments actually made into the enterprise.
Every hop needs its own paper
Hypothetical example: The file is a chain, and it is only as strong as its weakest link. Generating documents include sale agreements, corporate financial statements, employment or dividend records and tax filings. Movement documents include bank statements covering the whole period, transfer confirmations, and explanations for any currency conversion.
Where a loan is involved, the loan agreement and its security provisions decide whether the borrowed funds count as the investor's own capital at risk. The common failure is a gap: a transfer through a relative's account, a period where statements were not retained, or a business sale supported only by a summary. Each gap has to be explained with whatever secondary evidence exists, which takes longer and convinces less.
Collecting the records as the transaction proceeds is dramatically easier than reconstructing them afterwards.