Very good question!
At first I thought it must be in the manual of CapSense Tuner,
but I could not find any from PSoC 4 related CapSense Tuner Documents,
there may be some which I have not found though.
Then finally I find it in the "PSoC 1 and Capsense Controller - CapSense Data Monitoring Tools" document.
In the page 11, there is a table
So it was defined then and there.