What is the difference between surface piercing and dermal anchors?

A surface piercing has an entrance and an exit with a surface bar connecting the two. A dermal anchor is a single point piercing with only one hole in which the dermal anchor base sits. 

Neither of these piercings are “traditional” in the way a nostril or lip piercing is. Both come with a risk of rejection and in the case of dermal anchors rejection should be considered an eventually certainty.