The Sarira-Sariri tenet is central and exclusive to the Visishtadvaita philosophy. It explains the relationship between the Lord and the entire creation. The Lord is the Sariri and the entire creation comprising the chetana and the achetana beings and objects is His Sarira. As long as the chetana and achetana beings exist, He supports them and keeps them under His control. They cannot exist independently and are bound to Him seamlessly.
In a discourse, Aasuri Sri Madhavachariar drew attention to the Phala Sruti verse in the Vishnu Sahasranama Stotra that captures the essence of this bhava. The heavens, the moon, the stars, and all the heavy objects in the entire creation are kept in their respective places and supported and made to function owing to His Sankalpa. In the individual chetana and achetana beings and objects, the body exists as along as the jivatma exists. It becomes lifeless when the atma exits. Just as a jivatma directs his body, mind, intellect, etc according to his will, the Lord is the sole master. All others in creation are His possessions and exist to serve Him as Sesha bhutas. Brahma, Rudra, Indra, etc, are subordinate to Him. There is none who can equal His supremacy nor is there anyone who is above Him. This supremacy is His natural swabhava. When in life there are masters with servants to work for them, these are roles that are obtained by circumstances and for limited periods of time. But the Lord is the Master at all times and for the entire creation. It means no one can be His master and He is not obliged to serve anyone. Still, out of His own Sankalpa, as in Rama and Krishna avatars, He willingly plays the role of a Seshi, when He serves Vishwamitra and fights Tataka, or when He drives the chariot for Arjuna, or goes as a messenger for the Pandavas.