What a mistake apparent from the record is
Section 154 lets an obvious error in an order or intimation be corrected without the machinery of an appeal (Section 154). The key phrase is a mistake apparent from the record, meaning an error that is clear and does not need a long argument to establish: an arithmetic or clerical slip, a factual error, a TDS or tax credit that is in your records but was not given, a figure that plainly does not match your Form 26AS or AIS.
The boundary is important. Rectification is only for clear mistakes; it cannot be used to reargue a debatable point of law or a matter that needs investigation and two views. If your disagreement turns on interpretation, that belongs in an appeal, not a rectification. So the first question a CA asks is whether the error is genuinely apparent on the face of the record, because that decides which route you take.
The window, and the right to be heard
There is a time limit that catches people who wait. A rectification can be made within four years from the end of the financial year in which the order sought to be corrected was passed, and the department's system will not entertain a rectification of an intimation beyond that window. So an obvious error left unaddressed for years can become uncorrectable, which is why it is worth acting when the intimation arrives rather than assuming it will sort itself out.
There is also a protection built in. Under Section 154(3), before the department makes any rectification that increases your tax or reduces your refund, it must give you notice and a reasonable opportunity to be heard. So rectification is not a one-way tool the department uses against you without warning; it cuts both ways, and any adverse change has to be put to you first.
When to use rectification, and when to appeal
For an NRI, the practical value of Section 154 is speed and cost. A wrong TDS credit, a mismatch, an arithmetic error in a 143(1) intimation, these are exactly what rectification is for, and it resolves them far faster and more cheaply than a formal appeal under the appeal provisions, which is a longer, more formal process with its own timelines.
The judgement is matching the problem to the route. If the error is clear from the record, a rectification application, filed online, is the right and quickest fix. If the dispute is about how the law applies to your facts, a rectification will be rejected as raising a debatable issue, and you have to appeal instead. A practising CA reads the intimation, decides which route fits, and files it, whether a rectification to correct an obvious slip or an appeal where the point is genuinely arguable, all from the Indian side under your authorisation.