Let me tell you about my family first. I have my mom, father, 1 sister in japan (married, not a dependant anymore), 1 sister in canada (the one sponsoring us, married), 2 sisters again(dependants) and me(dependant)
Now, back in 2011 in the original application, the principal applicant is my mom, with 3 dependants (me and my 2 sisters here). My sister in japan was never included, my sister in canada was also not included since she is sponsoring us, my father was not included since he doesnt want to go to canada. In the imm0008 that we have submitted in 2015, the principal applicant(my mom) will be accompanied by me only, coz my 2 sisters(2 other dependants entered in 2011 application) doesnt want to go there anymore. So we wrote a letter telling them the reason why. I never entered my sister in japan in original application and in the imm0008, the same goes with my father i we did not include him in the original application coz he doesnt wish to go to canada, but i entered him in imm0008 coz it says on the form that i must include the spouse, but i checked the not accompanying box and told them the reason, the same goes to my 2 sisters(dependants that doesnt wish to canada anymore) i entered them in imm0008 but as a not accompanying dependants, in the letter that we wrote we explained there the reason as to why my 2 sisters arent coming anymore.
But i still received the letter from cic that some dependant is missing, i dont know who it is. And that accompanying and not accompanying dependants should fill the background declaration and additional family intormation.