Comment by tannhaeuser

Get a copy of The SGML Handbook! It includes the ISO 8879 text with much needed commentary. The author/editor deliberately negotiated this to get some compensation for spending over ten years on SGML; it's meant to be read instead of the bare ISO 8879 text.

Regarding HyTime, the editor (Steven Newcomb or was it Elliot Kimber?) even apologized for its extremely hard-to-read text, but the gist of it is that notations were used as a general-purpose extension mechanism, with a couple of auxiliary conventions introduced to make attribute capture and mapping in link rules more useful. sgmljs templating uses these to capture attributes in source docs and supply the captured values as system-specific entities to templates (the point being that no new syntax is introduced that isn't already recognized and specced out in HyTime general facilities). The concept and concrete notations of Formal System Identifiers is also a HyTime general facility.