The 15CB certifies the tax, not the source
A remittance out of an NRO account runs through the 15CA and 15CB regime (Section 195(6) read with Rule 37BB). The point most people get wrong is what the CA's certificate is actually about. Form 15CB certifies the tax position of the money being sent: what kind of remittance it is, whether it is chargeable to tax in India, at what rate, and that any tax due has been deducted or paid. It does not certify where the money originally came from.
So the gate to moving the money is tax, not history. If the funds are old savings and gifts on which nothing further is due, or income on which the tax has been paid, the certificate can be given. From FY 2026-27 these forms are renumbered 145 and 146, but the substance is unchanged. This is why a balance you cannot trace to its origin is usually still repatriable: the law is asking whether tax is clear, not whether you kept receipts from fifteen years ago.
How much you can move, and what sits outside the cap
From an NRO account you may repatriate up to USD 1 million per financial year out of your capital balances, and that pool is exactly where legacy money lives: old savings, gifts, matured fixed deposits, sale proceeds and inheritance, all counted together against the one-million limit under the RBI rules on remittance of assets.
Separately, your current income does not count against that cap at all. Rent, interest, dividends and pension, once the Indian tax on them is settled, are freely repatriable on top of the one million. So a large old NRO balance can usually be moved either within a single year's limit, or staged across financial years where it exceeds it, while the income the account keeps earning flows out separately. A practising CA maps which part of your balance is capital and which is current income, so the whole balance moves in the most efficient way.
When the source documents are simply gone
Where the original papers for old money no longer exist, the position is more workable than it feels. The tax law's gate is that the tax is clear, and the CA certifies that on the 15CB from the records that do exist, your account history, the interest already taxed, the nature of the balance. For the source itself, the standard route is a self-declaration by you, alongside the CA certificate, confirming the funds are your legitimately held NRO balances and that the one-million limit is not breached.
The honest point to flag is that how much source documentation a bank asks for is the bank's own compliance practice, not a requirement of the tax law. Different banks ask for different things, and some are stricter than others on decades-old balances. A practising CA prepares the tax certification and the self-declaration in the form your bank accepts, and where a bank is being unusually demanding, frames the position so the legitimately held, tax-clear balance is released rather than left stuck.