The body refers to the system's collective, physical form; the human within which the plural mind(s) are housed. Regardless of a system's plural identity, the body is definite and housed in reality.
Some endogenic systems may use multiple bodies via system travel or other similar methods, however this typically would only regard individual headmates travelling among bodies.
Systems may refer to their bodily identity as a "singletsona", considering their physical form to be in essence a different person entirely unrelated to the system. Many plural people wish to make a clear distinction between the body and the headmates, whether due to trauma relating to their body or any manner of other reasons.