A deeper discussion of the master valve function is appropriate. A master valve function can have different applications:
It can be used as main valve to prevent uncontrolled water flow in case a zone valve malfunctions and becomes stuck in the open position.
It can be used to activate a pump relay for those who use well water.
It can be used to selectively activate a fertilizer injector by zone. This allows variations in "fertigation" based on zone and plant types. In this application, the master valve function controls a valve to open or close the fertilizer suction line.
Most residential irrigation controllers do not have a master valve by zone feature (i.e. application #3) and only concentrate on applications #1 and #2. Application #3 is generally found on better professional level controllers.
I hope to see Rachio offer a master valve by zone feature. Better yet, I hope to see the capability to have more than just one master valve. For example my front yard has a valve set manifold and my back side has another valve set manifold. Each manifold requires its own independent master valve. I like to see each zone being able to selectively enable either MV1, MV2, MV3, etc. This is a wish list of functions that I'd like Rachio to seriously consider for future software upgrades.