Years of ordinary movement between personal and corporate accounts. The funds are lawful, but the path has to be evidenced hop by hop, and informal shareholder loan practices leave gaps.
Lawful is not the same as traceable
Owner-managers rarely have difficulty establishing that their money was earned legitimately; the business exists, it filed returns, the income is documented. The difficulty is the path. Funds that passed through a shareholder loan account, were lent to the company and repaid, or moved between a holding company and an operating company need each step explained with records.
Where those transactions were reconciled only at year end, the intervening detail may not exist in a form that reads clearly. The question to ask the accountant early is which parts of the history can be evidenced from primary records and which would have to be reconstructed. Knowing that at the outset allows the household to choose a funding route that is simplest to document, which is often not the route that is simplest for tax.