It is the percent of moisture found in that zone. The sensor reads the moisture level and reports it as a percentage. It goes from 0 to 100% - i then translate it to 0 - 1 and call the endpoint. Does this make sense?
For testing purposes I use this one (I did not want to spend too much just yet): http://www.amazon.com/gp/product/B0032WFD4G
But the http://www.koubachi.com/ works the same way.
I made a few extra calls to the /public/zone/:id endpoint to make sure I wasn’t missing it. Sadly as you found out, it was not there