High definition describes a camera that carries more pixels than standard analog video. The label covers 720p, 1080p and larger formats, and it says nothing about whether a face will be identifiable, because what matters is how many pixels land on the subject.
Resolutions on a box are easy to compare and rarely the reason a system disappoints. Pixel density at the distance you care about, lens choice and light are the real limits.
Resolution is not the whole picture
A wide 4K view of a parking lot can put fewer pixels on a plate than a narrow 1080p view of the same lane. Match the lens to the distance.
What to specify instead
Ask for the pixels per foot at each point of interest, the frame rate, and the light level under which the camera must still deliver usable detail.
Related:Commercial security camera systemsGlossary: Megapixel cameras