/paribhASA / परिभाषा

A /paribhASA (metarule) is a rule that explains how to interpret other rules.

As for instance, how do we know that AdguNaH replaces i इ with e ए? Because sthAnentarat... tells us how to interpret AdguNaH. That sthAnentarat... is a /paribhASA.

Some metarules are not mentioned by /pANini. As for instance, all grammarians agree that asiddhambah... ("in-stem rules ignore out-stem rules") is a general principle of interpretation of /pANini sUtras.

Why didn't /pANini mention it then?

My guess is that /pANini sort of applied it unconsciously and his students sort of gathered it intuitively from his classes, so he never saw any need of even WORDING it.

So if some smartie student had asked pANini "why can't I say pacAmaidam पचामैदम् applying this and that rule", he'd perhaps have answered, "hm, common sense perhaps? we have word-making rules and word-joining rules, the rules you mention aren't supposed to be used in that convoluted way, you are stretching their gospada गोस्पद too much".

But when some centuries later another smartie asked the same question, some prof considered it the question carefully, and saw that actually the rules do not forbid the pacAmai पचामै. Considering that as a failure of the rules, he concluded that /pANini took it for granted that wordjoiners work after wordmakers. And made the asiddhambah... aphorism to summarize his research. Not as something ADDED to pANini's work, but asa reconstruction of his original intent.