As flatten or spread concepts are quite confusing, this document will use below definition:

• spread body case: it means spreading a model using ..., e.g. op(...AModel)
• flatten body case: it means defining request body without @body and not using spread syntax ..., e.g. op(a: AModel)

If a body parameter is defined without @body, TCGC will create an anonymous model with spread usage for it, unless it is like case1, where there is only single spread body param, it will not create anonymous model, but reuse the spreaded model.