A Keystone (VRCP) holds several very different roles at the same time, all in one world, and does each of them well. Over the years, that world comes to rely on them for all of those roles at once.
The roles run side by side, and they have little in common. They all sit in one world, their work environment, so the people a Keystone meets in one role are often the people they serve in another. The parent met at the school gate on Monday may be across the council table on Wednesday.
In a regional orchestra, a cellist who also runs its programme for schools and represents the players on its board holds three roles in one building, every season. Each role can sit under a different employer, or under none, and it is still one world, because the same people turn up in all of them, whether the world is a town, a trade or a church community.
Keystones take each role to a solid standard and hold it there, and put the energy that polishing one role would take into keeping the others running. That is how three or four very different commitments stay in good order at once.
All of that grows up in one world, and it is worth most there. No single job title covers what a Keystone does, which makes them hard to replace inside their world and harder to explain outside it.