For grade A limit is less than 1cfu ! How it will be less than 1cfu ?
Suppose if you had exposed 5 plates and each had 1 CFU then the average will be 1 and according to the specification area fails in the viable count. The average of all exposed plates should be less than 1 . The average of all exposed plates should be less than 1 .
Grade A is a clean Area . Grade A area is a maintain according to USP - 34 <1116> Microbial Evaluation clean rooms so that is why Grade A acceptance criteria is a limit NLT 1cfu / Plate.
CFU= colony forming unit ( include total bacterial and Fungal count). Total aerobic viable count is not present in the Grade A area. that is why limit is less than 1cfu.