it's an online form served as digital signature of the principal applicant. you cannot submit an application without this form.what is the purpose of " Consent and Declaration " form. I saw this in the f12 trick in my account
if you want to know what's in it, you can create a new blank application(but don't submit)in PR portal and find this form at the bottom. I haven't checked word by word but it's very similar to the "consent and declaration" part of IMM0008 pdf version.
how they process this form is a complete mystery. i see many times when the status is returned (to request a form etc) this consent form is automatically voided. if you don't have a document request i don't think you need to worry about this.