Sutra:
105. "Aya" enkiRa viththAl, *senRAR kudaiyAm* enkiRapadiyE ellAvadimaikaLum seyya vENumenRu abEkshikkiRathu.

Meaning:
The fourth case (caturthI vibhakti) that is added to the nArAyaNa term expresses the entreaty to perform all kinds of service to Him.

Sri PBA Swami's Sarartha Deepikai:
As per Poygai Azhvar's pasuram (First Thiruvanthathi 53),

senRAR kudaiyAm irunthAL singAsanamAm
ninRAl maravadiyAm nIL kadaluL - enRum
puNaiyAmaNi viLakkAm pUmpattAm pulgum
aNaiyAm thirumARku aravu

which shows all the services provided by Sri Adisesha to the Lord, this fourth case ("Aya") that is added to the nArAyaNa term brings to light the desire to perform all kinds of service at His lotus feet.