Layers that contain one or more objects with a defined "Composite Level" greater than zero are considered composite layers. Composite levels make it easy to define the fill rules for objects without the construction of complex polygons or geometries. Essentially objects are drawn per layer in the order of their composite level, starting with 0 and increasing to a maximum composite level of 1000.

All "even" (and zero) composite levels are filled dark (ie. Paint), while "odd" composite levels are drawn clear (ie. Scratch).

See also

https://www.numericalinnovations.com/blogs/news/what-are-composite-layers