As you know, FFU=Fan filter unit
The interpretation of the wikipedia as follows :
A fan filter unit is a type of motorized air filtering equipment. it is used to supply purified air to cleanrooms, laboratories, medical facilities or microenvironments by removing harmful airborne particles from recirculating air. The units are installed within the system’s ceiling or floor grid. Large cleanrooms require a proportionally large number of FFUs, which in some cases may range from several hundred to several thousand. Units often contain their own pre-filter, HEPA filter and internally controllable fan air distribution.
In many cases, PBM backward centrifugal fan is commonly used in the FFU.