An informal grouping of multiple muscles and tendons along the back including the Trapezius (traps), Latissimus Dorsi (lats), Extensor Spinae and the Gluteus Maximus (glutes) that all attach along the spinal column.
They generally contribute to shoulder blade movement as well as support and mobility of the spinal column. The latter Gluteus Maximus is responsible for thigh movement, support of the pelvis and in biped characters/creatures is the primary benefactor for a erect upright gait.

If the back muscles are well developed and visible in the picture then it should be tagged.