It used to work when I tested it (pretty recent too).
Basically it takes a landscape wide texture.
Its ideal for RTs
And what it really does, is it let’s me change from grass to mud or ground at runtime when the … let’s not get into details, NDA and all.
But it did work, and it was a missing feature.
Output the mask you are using into the material base color to see what is up.
Needs clear black/white without fading to work right.