Agoniometer measures
a. a joint's maximum range of motion
b. a joint's minimum range of motion
c. a joint's flexion
d. extension and hyperextension

A goniometer measures A. A JOINT'S MAXIMUM RANGE OF MOTION.

Range of motion is the maximum distance and direction a joint should be able to move. In human body, each joint can be measure for its range of motion with the help of goniometer.

Each joint has a range of possible motion and range of actual motion. This range of motion can be hindered by swelling, stiff muscles,  cerebral palsy, rheumatoid arthritis, excess muscle or fatty tissue, age, and bone structure.