There are two types of zone valve references that address labeling in NFPA 99 (2012 edition). One is for new construction projects which is located here:

• 5.1.4.8.8 Zone valves shall be labeled in accordance with 5.1.11.2.
• 5.1.11.2 Shutoff Valves.
• 5.1.11.2.1 Shutoff valves shall be identified with the following:
  (1) Name or chemical symbol for the specific medical gas or
  vacuum system
  (2) Room or areas served
  (3) Caution to not close or open the valve except in emergency
• 5.1.11.2.2 Where positive pressure gas piping systems operate at
  pressures other than the standard gauge pressure of 345 kPa to
  380 kPa (50 psi to 55 psi) or a gauge pressure of 1100 kPa to
  1275 kPa (160 psi to 185 psi) for nitrogen or instrument air, the
  valve identification shall also include the nonstandard operating
  pressure.

The other reference is for existing facilities (which apply during projects such as the annual testing we perform), and that reference is located here:

(NOTE: The * next to Valves indicates that there is additional information available in the annex material. The annex isn’t “technically” enforceable however many inspectors do not seem to care because they enforce items out of the annex all of the time. Zone valves aren’t directly mentioned in this section of chapter 5, just in the annex of chapter 5.)

• 5.1.14.2.3.1 General. The elements in 5.1.14.2.2.2 through
  5.1.15 shall be inspected or tested as part of the maintenance
  program as follows:
  (8)*Valves — labeling

The * refers to:

• A.5.1.14.2.3.1(8) Additional inspections for zone valves include
  the following:
  (1) Locations (relationship to terminals controlled)
  (2) Leaks
  (3) Labeling
  (4) Housekeeping around alarm