Here's information from the in product help:

Select the province or territory where you lived as of December 31, 2014.

If You had residential ties in more than one province or territory, select the province or territory where you have your most important residential ties. For example, if you usually live in Ontario, but you were going to school in Alberta or staying in a ski chalet in Québec, you would use the package for Ontario.

Residential ties include where your home (owned or leased) and personal property are, and where your spouse or common-law partner or dependants reside. Other relevant ties may include social ties, driver's licence, bank accounts or credit cards, and provincial or territorial hospitalization insurance.

For more information, see Income Tax Folio S5-F1-C1, Determining an Individual's Residence Status.