This tool is a Digital Caliper (specifically the 150mm/6" model from Solar Wind), a precision instrument used to take highly accurate measurements of physical dimensions.
Key Functions: It is designed for "four-way measurement," allowing you to measure:External Dimensions: Using the large lower jaws for the thickness or outside diameter of an object.
Internal Dimensions:
Using the smaller upper jaws for the inside diameter of holes or slots.
Depth: Using the thin probe that extends from the end of the tool to measure hole depth.
Step: Using the back of the caliper to measure the distance from one edge to another on objects with different levels.