Much of this information is internal documentation. Care has not been made to make links work and make the documentation conprehensible for outsiders.
A marking menu is simply put another way to apply an instrument. Marking menues are associated with a give instrument upon initialization and then applied when a entry is chosen.
An example of this is CPNMenu.
Marking menues can also have dynamic behavior in form of dynamically chosing an instrument. This is fx. done in an entry in PlaceMenu.