Rolleiflex orange filter, variable optical density?

I purchased three color filters for my Rolleiflex 2.8F, and I'd like to check something before I go out to test it.

So, Green and Yellow filters has -1.5 optical density, but Orange says -1.5 -3. What does this mean? Why it has the range of 1.5-3? Is it variable? How do I meter for the orange filter?

Thanks!