14 CFR § 27.1555

Each cockpit control, other than primary flight controls or control whose function is obvious, must be plainly marked as to its function and method of operation.

For powerplant fuel controls—

Each fuel tank selector control must be marked to indicate the position corresponding to each tank and to each existing cross feed position;

If safe operation requires the use of any tanks in a specific sequence, that sequence must be marked on, or adjacent to, the selector for those tanks; and

Each valve control for any engine of a multiengine rotorcraft must be marked to indicate the position corresponding to each engine controlled.

Usable fuel capacity must be marked as follows:

For fuel systems having no selector controls, the usable fuel capacity of the system must be indicated at the fuel quantity indicator unless it is:

Provided by another system or equipment readily accessible to the pilot; and

Contained in the limitations section of the rotorcraft flight manual.

For fuel systems having selector controls, the usable fuel capacity available at each selector control position must be indicated near the selector control.

For accessory, auxiliary, and emergency controls—

Each essential visual position indicator, such as those showing rotor pitch or landing gear position, must be marked so that each crewmember can determine at any time the position of the unit to which it relates; and

Each emergency control must be red and must be marked as to method of operation.

For rotorcraft incorporating retractable landing gear, the maximum landing gear operating speed must be displayed in clear view of the pilot.