Field of Search Field (FOS)
To search the classifications that the examiner searched, and may or may not have listed in the patent classification field, use the FOS field code. FOS searches only the US Classifications in granted US patents.
US Applications do not contain any data in their FOS field.
FOS:370/254 --> Finds patents where the examiner searched in US class 370/254
FOS:370/254 NOT PCCL:370/254 --> Finds patents where the examiner searched in US Class 370/254 but did NOT classify the Patent in 370/254. This is helpful to expand your class searching for "scorched earth" prior art searches.