Depth is measured by length below the skin.
We get a release and line up that hair against our probe to see how deep to insert nearby hairs.
Although text books tell us how deep/shallow hairs are on different parts of our body, these hairs are moving targets as they transition in stages of growth.
I prefer working in the earlier stage of growth when hairs are deepest in the dermis. Once hairs have reached their maximum length, and this is when the hairs appear longest at the surface, they are more shallow under the skin. This means I have to work more shallow and I prefer working deeper.