On the Zeus model, NUCing (non-uniformity correction) happens automatically, or you can do it manually by pressing the control buttons at the 12 o'clock and 6 o'clock position at the same time. Sometimes you may want to do it before the unit does it automatically and so the option is there. For lack of a better description, consider it a sort of screen "refresh."
Some brands are manual, some are automatic. Some have multiple settings to accomplish the task depending on the firmware and the type of microbolometer inside.