Reference Posts are signs located in physical locations. They stay in roughly the same place over time and do not change when other sections of the road are realigned. This layer also contains current ALRS linear measures (LM) for the sign feature.

This is a Roads and Highways Event Layer - RP Stationing

For more information on the difference between reference post locations along the roadway and the ALRS mileage, please refer to the Linear Measure vs Sign Location informational page