Airpura Liner

Airpura unit liner is a mesh that wraps around the prefilter.  Sometimes called the sleeve.

For Black Units: The prefilter comes in white, this black liner mesh wraps around the prefilter so you can't see the white prefilter thru the intake holes around the walls of the unit. 

For White/Cream Units: This is simply to keep the material of the pre filter from showing through the intake holes.