I like the breakdown of social circles. You have your inner circle of closest friends, family or nobody. You have the next social ring outside of that- maybe your neighbors, (they may come over an eat dinner), an occasional friend that you see every couple of months when they visit. Then you have your outer social ring of perhaps your hairdresser who knows a good deal about you but you don't go out with her or socially call her. Your relatives that you aren't close to fit in this ring. So if you look at your social contacts as in series of circles, then it's easy to visualise. Now, you can have a close friend that may migrate out ,(they move away, or a misunderstanding). You may have a friend on your outer ring move up to a inner circle, (a misfortune brings you together, you find out you have something in common). l like the explantion of this, and it helps you understand friendships better.
The other idea about this, it helps with keeping boundaries, and acting appro with the level you are in. I think being on the spectrum, we don't always understand social standing and where we stand and why we can't be friends with certain people. It kinda has to do with where you yourself are in their ring. If you are in their outer ring, they won't be calling or inviting you over. You maybe upset but eventually you move on.
And as a final thought,sometimes people's first and second rings are so crowded that they truly don't have time to be a friend. So don't internalize and think you aren't an interesting person, it's just they have alot going on already.