Dear Yong Peng,

> The word kappa literally means rule, but I originally thought it to

mean "subject proper".

Kappa has several meanings. In the first introductory verse of
Kaccaayana there is the word 'susandhikappa.m" at the end. The Nyaasa
(Mmd p. 4) gives the following nibbacana on 'kappa' which is along the
lines of Aggava.msa's nibbacana on 'paccayaa' :

kappiiyati etthaati kappo. It is circumscribed or delimited at this
place, thus it is 'kappa'.

The .tiikaa (Mmd-p.t p. 24) glosses 'kappiiyati' with
'paricchijjiiyati' in the following comment:

kappiiyati etthaati kappoti ettha paricchedappadese attho aacariyena
kappiiyati paricchijjiiyaiti yasmaa, iti tasmaa kappo.

I don't fully grasp this comment nor the one before but it suggests to
me an area where the subject-matter is circumscrbed or marked out by
the teacher.

Of course we could go further and investigate and compare other
meanings of kappa (e.g your rule) but I think this is enough.

Best wishes,
Jim