Class 1 clean aisles must be narrow - 12 feet maximum. Class 1 return chases must be isolated from lower class cleanrooms and rated minimum Class 1,000. Selection of HEPA filters must be specifically for each class. The subfab allows for a more uniform laminar flow as well as a higher cleanliness level. Most modern and ultra-pure facilities are designed this way and have the price tag to prove it.
Some people consider low-wall return an important issue as opposed to returns in the ceiling.
Others argue the placement of HEPA filters and ceiling coverage is vitally important. The most important issue is not what Federal Spec 209E argues, but what specification the customer really needs to meet. A Class 10,000 cleanroom can meet the test of a particle counter for Class 1,000 if the door is left closed long enough. A room can clean up to almost any specification if particle tests are the only thing to worry about. Some clean areas in this range are for cosmetic reasons only.