In Canada, the term circuit venting has come back(from the 70's) with the 2005 National Plumbing Code, bringing with it a multitude of new rules required to properly circuit vent fixtures. A circuit vent is a vent that serves multiple fixtures on a horizontal branch, in a single story dwelling. Circuit vents must always be paired with a relief vent located downstream of the last circuit vented fixture and may also require an "additional circuit vent".
Circuit vents are connected to the fixture drain of the last fixture it serves and are sized by the largest trap served, overall length and total fixture unit load. The circuit vented branch itself is sized according to the trap sizes connected to it, but will be a minimum of two inches for small traps(1 1/4"- 1 1/2"), and a minimum of 3" for traps 2" or larger.
Relief vents are required for circuit vented branches to assist with air circulation in the branch. A relief vent may also be a fixture drain or a soil or waste pipe, but they are limited to the number of fixture units that can be drained into it and must be sized as a wet vent. The size of a relief vent is generally one size smaller than the circuit vent.
*The information contained herein is general information; please consult your area's plumbing code book for the appropriate information and rules for your area.