[NTG-context] How does lua callback work?

Dear List, I'm trying to intervene whenever context encounters a character that it cannot render in the current font. I guess this should have something to do with lua callbacks (pls correct me if I'm wrong), but the document http://www.pragma-ade.nl/general/manuals/cld-mkiv.pdf does not seem to elaborate on this. Does anyone know how the aforementioned task can be done, or how lua callbacks work in terms of the meaning of the concerned events and parameters? Thanks! Best, Sylvain