The Lord as Master
Śeṣitva is the quality of being the śeṣī — the master, the one who is served, the one in relation to whom all else exists as śeṣa (attribute, servant). While śeṣatva describes the jīva's quality of being an attribute/servant, śeṣitva describes the Lord's quality of being the one for whose sake the jīva exists.
Śeṣatva-Śeṣitva as a Pair
The pair śeṣatva (jīva's nature as servant) and śeṣitva (Lord's nature as master) describes the most fundamental ontological relationship in Vishishtadvaita. It is not merely a functional relationship (like that of an employee and employer) but a constitutive relationship: the jīva is what it is precisely in its relationship of śeṣa to the Lord. Remove the relationship, and the jīva has no coherent identity.
The Beloved Master
The śeṣī-śeṣa relationship in Sri Vaishnava theology is not cold or transactional. The Lord as śeṣī delights in the jīva (śeṣa), loves it, protects it, and desires its welfare more than its own. The relationship is modeled on the best of human relationships — a loving master who cares more about the servant's wellbeing than the servant does. This makes śeṣitva a quality of love, not domination.
Practical Expression
Understanding the Lord's śeṣitva changes how the prapanna approaches all of life: everything is the Lord's, everything is to be used in His service, and one's own identity finds its meaning and fulfillment only in this relationship. The goal of liberation is precisely to be fully and permanently in the relationship of śeṣa to the śeṣī — in Vaikuṇṭha, where this relationship is experienced without distortion.