Represents the total amount of unique identities in a certain period.
Each identity captured by a camera is counted only once in a given time frame. Meaning, if the same identity walks in front of the camera numerous times in a given time frame it is still counted as one unique visitor.
Example: if in observed period of 24 h, the same identity walks by the camera 15 times (15 sightings) he is still counted as one unique visitor.