The duty-free limit for residents returning to Canada is 1.5 L for wine only, 8.5 L for beer and ale only or 1.14 L for all alcoholic beverages.
The relief provisions for wine and alcoholic beverages in general can be found under heading 98.04 (9804.10.00 and 9804.20.00) of the Customs Tariff.
But I was unable to find where the exemption for a case of beer comes from.