The affix añ comes in the sense of 'product or part' after a word which ends in an affix having an indicatory ñ , such affix denoting 'its product or part'.,
The affixes having indicatory ञ denoting product or part, are अञ् 4.3.139, टलञ् 4.3.142, अञ् 4.3.154 वुञ् 4.3.157, ढञ् 4.3.159, अञ्, यञ् 4.5.168. When a tertiary derivative of a word ending with these affixes is to be made, the affix अञ् is used. Thus दैवदारवस्य विकारोऽवयवो वा = दैवदारवम्; so also दाधित्थम्, पालाशम्, शामीलम्, कापोतम् औष्ट्रकम्, ऐणेयम्, कांस्यम्, पारशवम् ॥
Why do we say ञितः ? Witness बैल्वमयं. Here मयट् is used after बैल्व and the derivative is formed by अण् of 4.3.136. So also बैदमयम् ॥,