The 5th case-affix (Ablative) is used when a word is governed by the following karmapravacanīyāḥ [[1.4.83]] i.e. apa , āṅ and pari,
Ex. अप or आ or परि पाटलिपुत्रादावृष्टो देवः 'It rained off or upto or with the exclusion of, Pataliputra.'
The परि here has the meaning of 'exclusion' 1.4.88 being read along with अप, therefore, not here वृक्षं परि विद्योतते विद्युत् 1.4.90.,