Hello:
the list you are talking about is a check list to make sure all required documents are enclosed with your application, it is like a reminder for you and makes it easier to asses once it reaches Mississauga.
You will check off all the boxes to confirm you are sending those documents. If there are any that you are not sending or that do not apply to you then you can write N/A beside the check box. That will let them know that is it not a forgotten document.
What I did for the income in the last 12 months was tally up all my pay stubs from December 11 to December 12(that is when I sent my application). I am not sure why they ask but I assume it is for those who apply before/after tax time and do not have the most recent option C.
Hope this helps!
Take Care
MadeInCanada