We maintain a fairly-complete HTML
describing PISM's internals. See this Browser if interested in extending PISM or to understand in greater detail the PISM configuration parameters, options, metadata conventions, and so on. Mostly it is automatically-built from doxygen-style comments in the source code, but it also contains these pages of general interest:
Also you can build the browsers yourself using PISM sources, whether stable or dev branch. For guidance on this, see the last page of the PISM Installation Manual or go to the development version page.