What does IRCC actually check when it says my duties did not match my NOC?

There is a published standard; it is short, and it is more specific than most of what gets said about it.
The two things an officer has to be satisfied about
The officer instructions set out one test in two parts, and both have to be met. The applicant must have performed the actions described in the lead statement for the occupation as set out in the NOC description. And the applicant must have performed a substantial number of the main duties, including all of the essential duties, of that occupation.
The lead statement is the paragraph at the top of a NOC entry describing what people in that occupation do. The main duties are the list underneath it.
That wording appears in the same form in the federal skilled worker instructions, last modified 5 March 2025, and in the Canadian Experience Class instructions, last modified 25 May 2023. It is the same test either way.
The part that is usually paraphrased wrongly
It is very often repeated as needing to have done all of the duties substantially. That is not what it says.
You need a substantial number of the main duties. But you need all of the essential ones.
Those are two different standards sitting in one sentence, and the second is the stricter. A person can have done most of the list and still fail if one of the essential duties is missing from their experience, and a person can be missing several main duties and still qualify if the essential ones are there.
Something that is not part of the test at all
Both pages say the employment requirements listed in the NOC description are not applicable.
The employment requirements are the education, credential, and licensing lines in a NOC entry. People spend a lot of worry on these, assuming that not holding a particular degree or licence disqualifies the experience. On this assessment, it does not. What is being assessed is what you did, not what the NOC says someone in that role usually needs.
What a reference letter has to contain
This is where the published material runs out, and it is worth knowing rather than guessing.
The general supporting documents page, last modified 23 April 2026, says a reference letter from an employer should come with your job duties, the dates you worked there, and your salary. Three things.
The officer instructions do not say what a reference letter must contain at all. Not on the federal skilled worker page, not on the Canadian Experience Class page.
Hours worked per week and the NOC code itself appear on neither, despite being on almost every checklist published by firms and consultants, and despite refusals turning on them. That does not mean they are a bad idea to include. It means the requirement is not published where you would expect to find it, and anyone telling you it is a published requirement should be able to show you where.
Why a letter can be true and still not work
Look again at what the officer has to be satisfied about. The actions in the lead statement. A substantial number of the main duties. All of the essential ones.
A letter that reproduces the NOC wording proves nothing about what you personally did. A letter that describes your role the way your manager would describe it in conversation may be entirely accurate and still leave an officer unable to see which of those duties you performed. Both can be honest letters. Neither does the job the test asks for.
The letter has to make your actual work visible against that structure, in your own employer's words, without inventing anything. That is a harder piece of writing than it sounds, and it is why this is one of the most common refusal grounds there is.
The reference letter is not the only evidence
The Canadian Experience Class instructions ask for employer letters of reference for every period of qualifying experience claimed. They also name other documents that are accepted: a record or letter of employment from the Canadian employer, work contracts, and pay stubs.
The same page says there is no obligation to provide a T4 slip or a Notice of Assessment, and that those documents are not conclusive on their own.
If your employer has closed, or your manager has left, or the letter you were given is thin, that list is worth reading properly before assuming nothing can be done. If you have never actually seen the letter that went in, because someone else filed the application, you can request the full file from IRCC: https://www.nextmovepath.com/community/how-do-i-get-a-copy-of-the-application-my-agent-submitted-for-me/
Where the burden sits
The instructions put it plainly. The onus is on the applicant to establish that they meet the criteria at the time of the application.
That is the sentence to keep in mind when deciding how much to include. An officer is not required to work out what you meant, or to look up your industry's terminology, or to assume that an unnamed task on your list was an essential duty. Anything that is not visible in the file is not in front of them.
Where this comes from
Federal skilled workers: Assessing applications against minimum requirements, officer instructions, last modified 5 March 2025.
Canadian experience class: Qualifying work experience, officer instructions, last modified 25 May 2023.
Your IRCC application: common supporting documents, last modified 23 April 2026.
Check the modified date on each before relying on anything here. And if you have already been refused on this ground, the wording of your own refusal letter decides what is worth doing next, so that is the document to get looked at properly rather than described from memory.