The sliding bevel is used to mark or check any angle. By sliding the beam from inside the stock you can adjust both components to set the desired angle needed.
The lock mechanism holds both components in place and can be a variety of mechanism types. These include wing nut, lever cam, slide lock or set screw. In use, you usually register the stock against a flat surface and then mark or check the angle against the beam.
You can set the sliding bevel by pushing the stock against one surface and adjusting the beam to the angle required. It can be used when duplicating an angle or even after the project is finished to check all angles. The beam can also be used as a guide when developing a knifewall.