Tax examinations rarely turn on disputed law. They turn on whether a taxpayer can evidence what they claimed, which makes record keeping the operative variable.
The burden generally sits with the taxpayer
In most systems, the person claiming a deduction or relief must be able to substantiate it. The authority does not have to disprove the claim.
That allocation means an entirely legitimate expense with no supporting record can be disallowed, and the taxpayer has little to argue with.
The practical implication is that documentation is not administrative tidiness but the substance of the position being taken.
Contemporaneous records carry more weight
Records created at the time of the transaction are more credible than reconstructions assembled after an enquiry begins.
A reconstruction may be accurate, but it depends on memory and inference, and an examiner is entitled to weigh it accordingly.
This is why the value of a record is set at the moment it is made, and why the habit matters more than the filing system.
Mixed-use items attract the most attention
Expenses with both business and personal character, such as vehicles, home working space and travel, require an allocation between the two.
An allocation is a judgement, and a judgement supported by a log or a calculation is defensible in a way that a round percentage is not.
Examiners concentrate here precisely because the documentation is usually weakest, not because the claims are usually wrong.
A short record kept consistently across a representative period is often accepted as the basis for an ongoing allocation, which is far less work than logging every instance indefinitely.
Retention periods outlast the filing
Authorities can generally open an examination for a defined number of years after a return is filed, and that window extends where returns were not filed or where significant omissions are alleged.
Records therefore need keeping well beyond the point at which they feel relevant, and the required period varies by jurisdiction and by the type of item.
Digital copies are widely accepted, though the requirement is usually that they be complete and legible rather than that they be original paper.
Bank statements are not a substitute
A statement shows that money moved and to whom. It does not show what was purchased or why it qualified for the treatment claimed.
Invoices, receipts and contracts supply the purpose, and it is purpose that determines the tax treatment of most items.
The same applies in reverse for income, since a deposit alone does not establish whether the money was a payment for work, a loan, a gift or a return of capital.
Anyone facing an examination is dealing with procedure that varies by jurisdiction and changes over time, and it is a point at which professional representation usually pays for itself.