The IMM 0008DEP is filled out by the Principal Applicant.
This form is for when you, the Principal Applicant, have more than 5 dependants. For up to and including 5 dependents you can use IMM 0008. However, for each additional dependent you must use an IMM 008Dep form. So if you six children, you have to complete one IMM 0008DEP and if you have seven children you have to complete two.
The principal applicant must complete this form on behalf of the dependents. It is completed just like the dependant portions of the IMM 0008 application which you've already completed.
As well, at the end you have a “consent to share information” and a declaration as the principal applicant who has filled out these forms that the information is correct.