I took delivery of my Bolt a couple of days ago and immediately set to going through the settings to see how the vehicle could be customized. KeyPass is such a prominent setting that it just needed to be explored, thus began my quest. It turns out, however, that there is virtually no public information on what it is or how to set it up. I spoke with my dealership, with Chevy technical support, reviewed the patents database, reviewed GM federal transportation safety board filings, etc. but all to no avail. I finally found an interview with the Product Marketing Manager from early 2016 where he explains what the feature is.
In summary, it is a feature for eventual car sharing applications. Let's say you wanted to rent a Bolt for a couple of hours. You log into an app on your smartphone, check local inventory, find one in a parking lot around the corner, download a security token (a KeyPass) and head over to the parking lot. As you approach the vehicle, the Bluetooth low energy beacon (BLE) in the vehicle activates the application on your smartphone which exchanges the KeyPass with the vehicle. If the KeyPass is authentic and validated by the vehicle, the vehicle flashes its lights to help identify itself and unlocks as you approach. This works from a distance of 200 feet or more based on how much interference is present surrounding the vehicle. The KeyPass application cannot start the vehicle, so I assume the actual key fob would need to be left inside the vehicle. At the conclusion of your ride, the KeyPass expires.
All in all this seems like a cool feature, but not something that is built into the myChevy app (or needed by it for it to perform its remote key fob functions). This is really designed for a use case involving a smartphone that has never been paired to the vehicle.