Yonin (jo)

There is yojutsu and injutsu in ninjutsu. Yojutsu is the way of entering an enemy territory by using a wise stratagem. Injutsu means infiltrating unnoticed by using a personal disguise. This volume will discuss the many ways and stratagems for infiltrating the enemy and gathering information. Therefore, yojutsu is [also] called yonin.

*The term ninjutsu is used here only in reference to the act of entering an enemy territory. Yojutsu could be any kind of infiltration that uses some sort of disguise (i.e., komuso, performers, fortune tellers, etc'). Injutsu is when the infiltration and information gathering is done completely unnoticed. For example, entering the enemy territory at night and hiding in the bushes during the day while observing the enemy's movements.