पर्याप्तिवचनेष्वलमर्थेषु

The affix तुमुन् comes after a verb having in composition with it अलम् and its synonyms when these words express to be capable of something.
The word पर्याप्ति means 'capability, ability, fulness'. Thus लिखितमपि ललाटे प्रोज्झितुम् कः समर्थः (Hitopadesa) 'who is able to avoid that which is stamped on his forehead'. लोकान् अलं दग्धुम् हि तत्तपः (Kumara II.56) 'his penance is able to burn the worlds'. अस्ति मे विभवः सर्वं परिज्ञातुं (Vikramorvasi 2) 'I have power to know every thing'. भोक्तुम् प्रवीणः कुशलः पटुर्वा 'skilful in eating'.

Another interpretation of the sutra is "the affix tumun is added to a verb, when it has in composition with it, the word पर्याप्ति or अलम् or a synonym of alam having the sense of paryapti. Thus पर्याप्तो भोक्तुम्, अलं भोक्तुम्, भोक्तुम् पारयति ।

Why do we say 'having the sense of capability'? Observe अलम्कृत्वा ।

Why do we say 'having the meaning of अलम्'? Observe पर्याप्तुम् भुङ्क्ते । The word शक् in the preceding sutra has not the sense of अलम् in that aphorism. With the sense of अलम्, the present sutra will apply: as शक्यमेवं कर्तुम् ।