Section 21.9 - Parts, subparts, and undesignated center heads(a) The normal divisions of a chapter are parts, consisting of a unified body of regulations applying to a specific function of an issuing agency or devoted to specific subject matter under the control of that agency.(b) Subparts or undesignated center heads may be used to group related sections within a part. Undesignated center heads may also be used to group sections within a subpart.