Hello, "Ref" means "reference level", on Earth "Sea Level". Isp Ref (SL) also affect the thrust of the engine. Most rocket engines have a lower Isp Ref. Some upper stage engines are designed to work properly only in vacuum, so in that case the Isp Ref should be set to a very low value.
On sites like Astronautix you have the two values : Isp (Vac) and Isp (SL).
On sites like Astronautix you have the two values : Isp (Vac) and Isp (SL).