According to the order of carrying out tasks and actions, several parts are divided into combat troops and marching troops. Usually each part is an echelon. For example, the first echelon, the second echelon, the pioneer echelon and the follow-up echelon. First echelon, second echelon, third echelon

The first echelon in a combat deployment or formation. When attacking, it is used to break through the enemy's defense; When defending, it is used to guard the first line of defense.

An echelon behind the first echelon in a combat deployment or formation. When attacking, it is mainly used to replace the first echelon, enhance the assault force and expand the results; When defending, it is mainly used to replace the first echelon defense, or to use the first echelon to counter the invading enemy and defend the deep position.

An echelon behind the second echelon in a combat deployment or formation. It is mainly used to strengthen the assault force or counter-assault, develop victory and achieve the ultimate operational goal.