This mode is based on Michael Wichura's TaBlE package for PlainTeX. The official manual for it is commercial (about 40 USD), see [http://www.pctex.com/books.html PCTeX] -- but note that the TaBlE manual only talks about the original syntax, which does not use <cmd>NC</cmd>, <cmd>HL</cmd> cum suis.
The only ConTeXt docs are in [[manual:ms-cb-en.pdf|ConTeXt - an excursion]]. There is also
an introductory article in tugboat [http://tug.org/TUGboat/Articles/tb28-3/tb90mahajan.pdf ConTeXt basics for users: Table macros] by Aditya Mahajan (2007).