A panel gauge has a large pin / spur through the end of the stem to mark the sheets of wood.
Panel gauge fence
The panel gauge, because of its longer stem, also needs a large fence to help keep the tool steady against the work surface.
The fence is usually around 228mm (9″) long.
Panel gauge stem
The stem of the panel gauge is what allows the tool to reach over the surface to mark it in the correct position. The stem is much longer than on other marking out tools and this is what allows it to mark larger sheets of wood.
Panel gauge thumb screw
The thumb screw of a panel gauge is located on top of the fence. When turned clockwise it locks the stem in position at the length required by the user. The thumb screw is turned anti-clockwise to release the stem.