Sergeant first class and Sergeant

Sergeant first class noun - A sergeant in the Army above the rank of staff sergeant and below master sergeant.

Sergeant and sergeant first class are semantically related. In some cases you can use "Sergeant" instead a noun phrase "Sergeant first class".

Sergeant noun - Any of several noncommissioned officer ranks in the Army or Air Force or Marines ranking above a corporal.

Sergeant first class and sergeant are semantically related. Sometimes you can replace phrase "Sergeant first class" with "Sergeant", this terms are similar.