There are many types of indicators, differing in terms of build and application. Presented in this category are: dial indicators, electronic indicators and 3D sensors.
The most common dial indicator is used to check, for example, if the shape of an object is correct, if the relative position of machine parts is correct or to ensure proper mounting of a workpiece. It measures the exact shift of the indinctor's stem that is touching the object's surface and checks if there are any irregularities or inaccuracies in relation to set nominal dimension or only to adjacent positions. In dial test indicators the stem is replaced by a swinging arm. Maesurment is made by indicating it's deflection in relation to the object.
A dial indicator is usually mounted on a stand and it's set in a desired position in which the gauge head touches the measured surface making the indicator's needle slightly deflect.
Electronic indicator has the same application. The difference is it measures the shift using an electronic system.
Another group of indicators consists of edge finders and 3D sensors. They are designed to establish the position of the object according to defined program of the CNC machine. They indicate (as with dial indicators) the shift of the stem on a dial. Edge finders work on two perpendicular axes on a surface perpendicular to the main axis of the tool itself. 3D sensor additionally works on a parallel axis (usually vertical).