Most Canadian immigration applications (and some pre-application processes) ask you to list your employment history. While it may be difficult for you to quickly find a record of your employment going back several years, it's easy to make a mistake if you go by memory. And mistakes can be costly.
Take the scenario where you are creating an Express Entry profile.
If you are selected for an Express Entry draw, you will need to supply the documents to prove your employment when you apply for permanent residence.
If you are selected for an Express Entry draw, you will have very little time to gather these documents -- there is a short window to apply after an invitation, and preparing an application is a lengthy process, so for practical purposes you will have a week or two to finalize all documents. This window can and has changed, for example, changing from 90 to 60 days in 2021 (https://www.canada.ca/en/immigration-refugees-citizenship/news/notices/changes-invitation-apply-period-express-entry.html)
If you are unable to gather documents in time, you will have to document how you tried to gather them -- this means documenting calls you made, emails you wrote, letters you wrote, and responses you received where people denied your requests. If any communications are in another language, those communications will need to be translated by an official translator.
If you are unable to gather documents in time, IRCC may deny your application, even with an explanation.
If the documents you gather have information that differs from or contradicts your Express Entry profile, IRCC may deny your application.
If the documents you gather have information that differs from or contradicts your Express Entry profile or if you are unable to gather your documents on time, you will have to explain why the information is different.
In the worst case, if the documents you gather have information that differs from or contradicts your express entry profile, IRCC may consider you to have materially misrepresented facts, and you both would be considered inadmissible, have to leave Canada, and be unable to apply again for 5 years (as of June 2026), a penalty that could change in the future (for the worse).
So rather than submitting your Express Entry profile or other immigration application from memory, take the time to verify your employment with documents you have saved, or contact former employers to request a letter verifying your prior employment. Keep these records in a file, because you will need to come back to them later.
IRCC has a list of documents that you can use to confirm your current and past employment (as well as other common supporting documents) here:
In the case of employment, the easiest and most direct proof is a reference letter from your current and prior employers.
Reference letters must be written on company letterhead, show the company's full address and telephone and fax numbers, and be stamped with the company's official seal (if the company has one).
The letter should include all of the following information:
the specific period of your employment with the company (the date you started working and the date you stopped working)
the positions you held during the period of employment and the time spent in each position (the number of hours per week in each position, and the start and end dates in each position)
full details of your main responsibilities in each position
the 5-digit code that your employer would use to classify each position under Canada's National Occupation Classification (NOC) - see https://noc.esdc.gc.ca/
your total annual salary plus benefits for each position
the signature of your immediate supervisor or the personnel officer at the company
a business card of the person signing
Notice also what the reference letter does not include -- a rating of how well you did the job. Some employers, for liability reasons, instruct managers not to give verbal or written references. You may want to approach these employers and ask them for an "employment verification letter" instead of a "reference letter."