The October 7, 2021 edition of the Aeronautical Chart Users’ Guide notes that IFR en route charts now show expanded VOR standard service volumes that are being implemented to support the minimum operational network (MON). That network is a key part of the FAA plan to reduce the number of ground-based navigation aids (for more information see Latest on VOR Decommissioning and related posts here at BruceAir).
The new ACUG includes the following note:
IFR ENROUTE CHARTS
Two new VHF Omnidirectional Radio Range (VOR) standard service volumes (SSV) have been implemented in order to achieve VOR service within 70 nautical miles above 5,000 feet above ground level (AGL). The new NAVAID codes are VOR Low (VL) and VOR High (VH). Along with that effort, two new distance measuring equipment (DME) SSVs of DME Low (DL) and DME High (DH) have been implemented to support DME-DME RNAV service. Legacy SSVs of Terminal (T), Low (L), and High (H) will continue to be maintained.
In the past, NAVAIDs at one location typically all had the same SSV. For example, a VORTAC typically had a High (H) SSV for the VOR, the TACAN azimuth, and the TACAN DME, or a Low (L) or Terminal (T) SSV for all three. A VOR/DME typically had a High (H), Low (L), or Terminal (T) for both the VOR and the DME. A common SSV may no longer be the case at all locations. A VOR/DME, for example, could have an SSV of VL for the VOR and DH for the DME, or other combinations.
See Charting Notice ENR_21-01_CN_SSV_Codes for more information.
Sample VOR/DME NAVAID box with two SSVs: