The affix map comes always after the words ending in the affix tri, in the sense of 'completed thereby'.,
The त्रि here refers to the affix 'ktri' 3.3.88. Thus the root डुपच् takes त्रि and forms पक्त्रि, which is not a complete word, but must take the augment मप्, to form a full word. Thus पच् + क्त्रि + मप् = पक्त्रिमम् 'what is completed by being cooked'. So from डुवप् we have वप् + क्त्रि + मप् = उप्त्रिमम् (VI. I. I5): from डुकृञ् we have कृत्रिमम् ॥
The word नित्य in the sutra indicates that the affix क्त्रि never comes singly by itself, all words ending in 'Ktri' are invariably followed by मप् also. In fact मप् way be regarded as an invariable augment of the affix 'Ktri'.
Vart:- After a word ending in an affix denoting 'condition', the affix इमप् is added. Thus पाकेन निर्वृत्तं = पाकिमम् (पच् + घञ् 3.3.18 + इमप्), so त्यागिमम्, सेकिमम्, कुट्टिमम् ॥,