COB　(Chip On Board) stencil is used for printing on the PCB with pre-mounted devices (such as bare chip ) or PCB with some convex parts on its surface . This can be helpful when you want to avoid the contact with such pre-mounted devices or some convex on the surface, and at the same time you want other parts being flat condition in printing.
This stencil is produced by electro-forming method , made of special Nickel Alloy( for this hardness, the stencil has long durability without crack or chip-off )