Sometimes and unfortunately, if you don't follow Checklist strictly, Applications will be returned by IRCC unprocessed. (under Section 12 and Section 87.3 of the Immigration and Refugee Protection Act (IRPA))
Section 12 relies on Section 10 and Section 11, therefore you should read it thoroughly.
Forms required by Part A of Document Checklist IMM 5533 are needed to satisfy section 12 of the IRPR, whereas the supporting documents required by part B of the Document Checklist IMM 5533 must be included to meet the conditions established by the Ministerial Instructions issued on 15th December 2016 (MI20).