A crib sheet I found on the Internet a few years back (and apparently no longer up) says:

#11, X1 (060)
A light yellow-green filter which corrects panchromatic film to tungsten eye response. Differentiates green tonal values in landscapes and reduces distant haze. The filter factor is approximately 2.
An X2 would be stronger. I haven't used either in recent memory, so can't add much.