A vent stack is a major supplier of air for a drainage system and is commonly used on multi-story soil or waste stacks. Vent stacks must either terminate to outside air or be tied into a vent header. Generally any building that has a drainage stack over four stories high must have a vent stack unless it is acting as a wet vent. A soil or waste stack serving water closets on multiple floors cannot be wet vented so a vent stack is one option to allow for adequate air circulation in the drainage system. Your local code may allow fixtures to be drained by the vent stack as long as it is sized as a wet vent, and only on the lowest floor of the vent stack.
Sizing vent stacks
Vent stacks are sized by the number of fixture units that are being served by the soil or waste stack and also by the length of the vent stack. Any other vent that is connected to a vent stack will also be added to the fixture unit load of the vent stack and the size will never be less than half the size of the bottom of the soil or waste stack it serves.