My experience with tablets and smart phone apps as well as my own habits tells me that hardly anybody writes user manuals or help files any more because nobody reads them! They are becoming the dinosaurs of present technology. It’s not that they aren’t needed for some complex items, but terms like this shouldn’t be a mystery for native English speakers
If the parameter WATER LEVEL was labeled as % WATERING or some other verb form, then most native English speakers would quickly comprehend, what it refers to.
I can sympathize with the OP because I had the same question, and I’ve been around irrigation controllers for a while.