Has likely been mentioned but I couldn't find it in the documentation or a search. I put the variable name with and without a space so others can search for it it they wish.
Thanks
John
It probably depends on the driver, but in the stock drivers I'm aware of (mostly the Generic Z-Wave CentralScene dimmer published as an example), it is one mechanism the driver uses to try to track physical vs. digital events for the purposes of noting this on the generated event for attributes like "switch" and "level." Like most state variables, it just is used internally by the driver code and generally not of much concern to end users, though I do understand the curiosity. 
It got me curious because I've never noticed a bin that was not -1 .
