To function is to support a specific system.
A function is a process which supports a specific system.
Function is a crucial systemic concept, because all systems develop from related functions which create emergent processes.
Functions can always be defined in relation to supporting identified processes or systems, even if they create new elements. For example, actors can support a process by creating resources which serve as inputs in that process. (Bone marrow supports the circulatory system by creating red blood cells; etcetera.)