NAVAID Classes

VOR, VORTAC, and TACAN aids are classed according to their operational use. The three classes of NAVAIDs are:
  1. T- Terminal.
  2. L- Low altitude.
  3. H- High altitude.

Note: The normal service range for T, L, and H class aids is found in the AIM. Certain operational requirements make it necessary to use some of these aids at greater service ranges than specified. Extended range is made possible through flight inspection determinations. Some aids also have lesser service range due to location, terrain, frequency protection, etc. Restrictions to service range are listed in Chart Supplement U.S.

Source: Pilot Contoller Glossary (PCG)