A sergeant first class is typically assigned as a platoon sergeant at the company level or battalion operations noncommissioned officer in charge at the battalion level, but may also hold other positions depending on the type of unit. In a combat arms role, a sergeant first class is typically in charge of from 18 soldiers and four tanks in an armor platoon to 40 soldiers in a rifle platoon. A sergeant first class' primary responsibility is training and mentoring lieutenants, tactical logistics, tactical casualty evacuations, and being senior tactical advisor to the platoon leader. With today's operations tempo, a sergeant first class may fill the role of platoon leader if no suitable commissioned officer is available. Sergeant first class replaced the now defunct rank of technical sergeant in 1948 (although the recently separated U.S. Air Force retained the rank of technical sergeant at the E-6 level).
A sergeant first class is addressed as "sergeant" except in certain situations; such as cannon artillery units, where a sergeant first class serving as platoon sergeant is commonly referred to as "Smoke". If a sergeant first class is appointed to fill the role of first sergeant, he or she is addressed as "First Sergeant". Typically a sergeant first class assigned on a manning document to fill a first sergeant role while being promotable to master sergeant can be frocked to first sergeant rank and hold the insignia due its position.
Sergeant first class is the first rank in the U.S. Army to be selected by the centralized promotion system. As such, it is considerably more difficult to achieve than the previous ranks. A sergeant first class is considered the first senior non-commissioned officer, and gains not only noticeable but several benefits due to the position. For example, a sergeant first class cannot be demoted by standard non-judicial punishment.
Israel Defense Forces
Rav samal (רב-סמל) or rasal (רס"ל), sergeant first class, is the lowest non-commissioned officer (נגדים) rank in the Israel Defense Forces (IDF), above samal rishon, סמל ראשון (samara, סמ"ר), staff sergeant. Because the IDF is an integrated force, it has a unique rank structure. IDF ranks are the same in all services (army, navy, air force, etc.). The ranks are derived from those of the paramilitary Haganah developed to protect the Yishuv during the British Mandate of Palestine. This origin is reflected in the slightly-compacted IDF rank structure.