A subrenderer is a special Lua function that renders the large area in the lower-right corner of the inventory screen. In the default code provided in base/, subrenderers are used to create the chest and scroll, and concievably about anything else could be built.
See the stats and inventory screen renderers in base/render.lua for more information on subrenderers. (Another place to look is the implementation of chests and scrolls in base/objects.lua)
By default, a subrenderer is called 6 times a second, as well as any time it is clicked or interacted with.
Any efforts to reconstruct the Amiga CSB magic map are quite welcome.