When we look for dermatomes in the body which should we remember – principal dermatome or sensory dermatome, and what is the difference between the two? (Physical therapy course manual book by IER?)

A myotome is a group of muscles innervated by a single spinal nerve root. Similarly, a dermatome is an area of skin innervated by a single nerve. Anatomically, a myotome is the motor equivalent of a dermatome.

Principal dermatomes mean the same thing with sensory dermatomes. I would recommend that you know all the dermatomes. It is also helpful to remember that a single spinal nerve root, say C6 for example, may refer to elbow flexion/wrist extension in resistive testing/myotomes, to the brachioradialis in reflex testing and also to the lateral forearm and thumb in dermatomes.