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As per CRA, electing under section 216 allows you to pay tax on your net Canadian-source rental income instead of on the gross amount. If the non-resident tax the payer withheld is more than the amount of tax payable on your section 216 return, the CRA will refund the excess to you.
Please visit the article on T4144: Income Tax Guide for Electing Under Section 216 – 2020