Here is the explanation I found related to IRCC File number:
How to find your file number
File number is a unique identification of your immigration or visa case. This number is always used when CIC communicates with you.
You may find this number on the first page of the document CIC sends to you. Very often, this number is quoted in an email sent to you from CIC.
If your visa application has been rejected, your file number would be written at the top of the first page of rejection letter.
Here are a few examples of file numbers: V123456789 – visitor visa application (Temporary resident visa or TRV); W123456789 – Work Permit application, S123456789 – Study Permit applications. Permanent residency application numbers usually start with B or E (EP for Provincial programs) or F for Family Sponsorship applications.