Digests are off by default. Each user needs to explicitly turn it on to whatever setting they prefer.
What Digests does is to capture the immediate notification triggered by bbpnns and save it to a queue. If the user selected to also receive immediate notifications, it lets it through – otherwise it deletes the user from the notification list.
Then, every so often, it will read the queue and notify users of the messages depending on their frequency settings.
That’s why there’s no option to choose which roles Digests works with. It doesn’t really care – it simply acts like a goalkeeper, storing away the balls.
If you can explain to me how exactly you want Digests to behave, I can probably cook up a filter to adjust it for you.