First you should know that the Government of Canada (federal) has final say on immigration matters. Quoting the Quebec Immigration page on this
The Government of Canada has jurisdiction over the final admission of
applicants. After obtaining your Certificat de sélection du Québec
(CSQ – Québec Selection Certificate), you must submit an application
for permanent residence (visa) as a skilled worker selected by Québec
to the Centralized Intake Office in Canada, in Sydney, Nova Scotia.
Looking on the glossary page of the CIC, a principal applicant is simply the main person applying and the one who bears the PR fees. It has more significance if you have children.
Here's a few definitions from their page for more info
Principal applicant: When a family applies together, one member must
be the main or “principal” applicant. For example, a mother applying
for permanent residence with her three children would be the principal
applicant. When parents are included in an application, dependent
children cannot be principal applicants.
Accompanying family member (Related term: Accompanying dependant):
A spouse, common-law partner, dependent child or dependent child of a dependent child (grandchild), who plans to immigrate to Canada with the principal applicant. Accompanying family members are included on the
Right of permanent residence fee: A charge paid by a principal applicant (with some exceptions), and a spouse or common-law partner
travelling with them, before the applicant can become a permanent
resident of Canada.