There are two auxiliary inputs (purple ports) in the zMatte node. The first is a Holdout Matte and the second is a Garbage Matte.
From the user guide:
Holdout Matte/Garbage Matte
A Holdout Matte defines foreground areas that should not be keyed out, while a Garbage Matte defines background areas that should be keyed out. zMatte has Holdout Matte and Garbage Matte auxiliary inputs that can come from a source image or a node like Roto.
- If there are foreground areas that are not keying properly, hook a Roto node into the Holdout Matte input. Roto the appropriate foreground areas and they will be added to the output matte.
- If there are background areas that are not keying properly, hook a Roto node into the Garbage Matte input. Roto the appropriate background areas and they will be subtracted from the output matte.