The affix ḍāp comes optionally after both - 1. prātipadika (Nominal Stem) ending in an 2. bahuvrīhi samāsa ,
Of the affix डाप्, the letter ड् indicates that the affix is to be added after the elision of the टि 1.1.64 of the base. Thus पामन् + डाप् = पाम् + आ = पामा, This being an optional rule we have I. s पामा d, पामे or पामानौ, pl, पामाः or पामानः; similarly with सीमा, as, सीमा, सीमे or सीमानौ, सीमाः or सीमानः ॥
So also in Bahuvrihi compounds ending in अम्. As:-
बहुराजा, बहुराजे or बहुराजानौ, बहुराजाः or बहुराजानः ॥
बहुतक्षा, बहुतक्षे or बहुतक्षाणौ, बहुतक्षाः or बहुतक्षाणः ॥
Why do we say \optionally\ ? So that the option may apply to sutra 4.1.7 also; i. e. when a pratipadika ending in वन्, which can be regarded as ending in अन्, is a Bahuvrihi compound, then the change of न् into र् and the application of ङीप् are optional we may apply the affix डाप् instead. As:-
बहुधीवा or बहुधीवरी; बहुपीवा or बहुपीवरी ॥,