Your province or territory of residence is the province or territory
where you lived or of which you were considered to be a
factual resident on December 31.
http://www.cra-arc.gc.ca/tax/individuals/topics/income-tax/return/completing/personal-info/province/menu-e.html
You usually have to use the tax package for the province or territory where you resided on December 31. If you had residential ties in more than one province or territory, use the package for the province or territory where you had your most important ties.
Residential ties means where your principal residence and personal property are, where your spouse, common-law partner, or dependents reside, and ties that may be relevant include social ties, hospitalization insurance, bank accounts, and driver’s licence.
For military members who may have been employed at several locations during the year this may be somewhat confusing if you do not have spouse or children, are living in single quarters, and all your personal property accompanies you even if you're sent temporarily to another location. Factors you should take into consideration are;
- Your parent unit, (on TD or even attached posting you still remain on the strength of your parent unit)
- If you are considering using your parents home as residence of record, think, do you still live there.
- If you use as your residence a province in which you did not receive employment income while being employed for the full year a red flag may be raised at CRA.
A tax credit for rent is usually a provincial tax credit and applicable only if you are resident in that particular province. Not all provinces have such tax credits.
Some may try to use the province that will give them the largest refund, but the best suggestion is you should never try to fool the taxman. Once the new income tax filing season starts, CRA will have hotlines available that can answer this question and any other question you may have.