tnx for your kind explanationNot the actual letter. Where you enter your work experience in your express entry profile. I'll give you an example.
Say I selected my main Noc as A and then in the work history section I listed my jobs with Noc A. But then I decided to change to Noc B, but didn't update my work experience to reflect NoC B as well. The system will see that I have no experience in Noc B and will flag the profile as ineligible (since I believe you need to have at least 1 yr experience in you chose noc).