Well yes, ahem. As it turns out, our fidelity agreement has some details and I think one of the reasons we have the agreement is so that we all know what details to expect.
So we have a three-person unit (a V), and we have formally agreed not to have sex outside of that unit. This means (we hope) that if one of us got swept up in a relationship with a new heartthrob, sex with that heartthrob wouldn't just happen, we are committed to exercising enough self-control to abide by our agreement.
But we also agree that our three-person unit could morph into a four-person unit. If it did, the new person would no longer be off-limits sexually. But there are more restrictions about how that would all come about. Seeing the new person in the first place couldn't happen in secret, on the contrary the progress of the new relationship would have to be shared with the rest of the V. The new person would have to make themselves available to meet the rest of the V rather soon. Before the new person could become part of our poly unit, the three of us would have to like the new person a lot, and the new person would have to like all three of us a lot. And (before becoming our "fourth person"), the new person would have to agree to the rules I've described in this post.
Sounds pretty stringent, I know. Just keep in mind that expanding our unit (with a fourth person) isn't high on our priority list. None of us are dating right now, and none of us are looking. And it seems unlikely that that state of affairs will change. It could change, hypothetically, but most likely, a three-person unit is what we'll continue to be, "til death do us part." Because we're not interested in seeking a fourth person, we don't mind the stringent agreements. And I think one of the reasons we have the agreements (formally) is so we have a solid amount of organization around how we'll do things. We could of course decide to change our agreements, but so far it doesn't look like we're interested in doing that either.
We could in theory become a five-person unit, a six-person unit, or what have you, but the basic rules/principles would remain the same. Each new person would have to jump through the same series of hoops. To us, I guess, our V is like a rather exclusive club. But I don't want to be misunderstood, we aren't snooty about it and don't look down on RA or poly in general. We just do what we do because it works for us. Though I guess I can see how it sounds snooty.
Anyway I'm an open book; if you have further questions just ask away.