From a post above.
408.4 Field Identification Required.
(A) Circuit Directory or Circuit Identification. Every
circuit and circuit modification shall be legibly identified as
to its clear, evident, and specific purpose or use. The identification
shall include an approved degree of detail that
allows each circuit to be distinguished from all others.
Spare positions that contain unused overcurrent devices or
switches shall be described accordingly. The identification
shall be included in a circuit directory that is located on the
face or inside of the panel door in the case of a panelboard
and at each switch or circuit breaker in a switchboard or
switchgear. No circuit shall be described in a manner that
depends on transient conditions of occupancy.
(B) Source of Supply. All switchboards, switchgear, and
panelboards supplied by a feeder(s) in other than one- or
two-family dwellings shall be marked to indicate each device
or equipment where the power originates.
See bold underlined type above.