Caliper is a tool for logging the tubings inside. It provides information of the inner diameter and then tells you the state of the tubing due to restrictions or corrosion.
It’s got multiple metal fingers which follows the tubing wall as the caliper is being pulled upwards.
The caliper is usually run with e-line, but there is also memory calipers, run by batteries.
It’s important not to run in hole when the caliper is activated due to it’s fragile fingers. They are only for running out of hole.