The September 8, 2022 edition of the Aeronautical Chart User’s Guide notes that:
Beginning with the September 8, 2022, publication date, all Standard Service Volume designations will be shown for VOR, VOR/DME, VORTAC, DME, and TACAN NAVAIDs on both the IFR Enroute Low and High Attitude charts.
Publishing this information is part of the FAA plan to implement the Minimum Operational Network (MON) of VORs (see also AIM 1-1-3-f), which will leave some 589 VORs still in operation after the program is complete, now scheduled in FY2030.
As the FAA decommissions VORs, mostly in the East and Midwest, it is expanding the service volumes of the navaids to help ensure signal coverage for aircraft not equipped with GPS or during GPS outages. The table below, from the Aeronautical Chart User’s Guide, shows both the new service volumes and the codes used on IFR charts to identify the service volume associated with each VOR.
Here’s a section of a low-altitude IFR chart that shows navaid information boxes with some of the new standard service volume classifications.
You can find more information about the new VOR service volumes in AIM 1−1−8. NAVAID Service Volumes.