I was unable to find a better word to describe it since there is no public checklist that tells someone how likely they are part of a certain wave, which is why I put it between single quotes (could have probably elaborated on it a bit).
What I am essentially trying to say here is that RDC is an investment from Roblox where engineers get feedback to help them develop better features and developers get the opportunity to learn more and create more connections. If I am a completely new developer who does not know how to program, has barely touched Roblox Studio and is definitely not going to release a game within the next year, then there is not much reason for the developer to have access to the resources at RDC. It makes more sense to invite the developers that show promise, are already established and will benefit the most from RDC.
So in short, the relevant users would be users that can help the engineers figure out what features are best to prioritize in the next year and will also benefit from being at the event. That being said, this is all from my personal experience and from what I’ve heard from other users, so there might be more to the invitation process. Maybe I’ve been receiving invites each year because I have funny hair, who knows!