IMM0008 is for the (principal) applicant and dependents to fill out. Your spouse (if any) and any children under the age of 22 (if any) can be dependents and included on the form. Any adult children would not be put on this form. No Canadian citizens or PRs on this form, because anyone on this is basically an applicant (or a non-accompanying potential applicant). Basically only the principal applicant fills one IMM0008 out, unless there are a whole lot of kids or a few other specific scenarios (kids of kids? can't recall). Probably doesn't apply so not going to look it up.
IMM5406 is complete family info - additional family info. You list everyone whether dependents or not, whether Canadian citizens or PRs or not. The principal applicant completes one, as do anyone else on the IMM0008 who is over 18 (spouse, adult children under 22).
Assuming a fairly simple scenario: you are married and all children are adults > 21 years of age. You fill out IMM0008, include your spouse but no children, and both you and spouse complete the IMM5406, which will have a certain amount of overlap of the family members.
