In Washington DC there is a Mass Transportation Benefit Program for many government employees. The FAQ at the bottom of the page says
Are my personal funds separated from my transit benefits on the same card?
Yes. SmarTrip® cards linked to a transit benefit account store funds in two separate "purses:"
So when riding the metro as a MTB program user, how do you select which purse to spend from? The rules are pretty clear that I can only use the MTB for my commute, how does the scanner know if I am commuting?
This WMATA link suggests there are three different purses, transit, parking, and personal, but not how to use them