Bhagavanta.m viijamaano ti thero vatta-siisena .thatvaa Bhagavanta.m
viijati. (DA 2.517)

Bhagavanta.m biijayamaanoti thero vattasiise .thatvaa Bhagavanta.m
biijati. (CSCD 2.96)

This sentence appears in Buddhaghosa's commentary on the
Mahaaparinibbaana Sutta, i.e. within the Suma.ngalavilaasaii. Dr.
Yang-Gyu An translates it as follows:

Fanning the Blessed One: the Venerable Ananda stood fanning the
Blessed One as his duty.

My question concerns the word 'vattasiisa.m'. Is it a avyayiibhaava
compound, more literally meaning (as inflected in this case) 'under
the heading of duty', or is there some other nuance attributable
to 'siisa.m' that I am unaware of?

Any help would be appreciated.

With best regards,

Joseph Dawson