From A to B? - Or From B to A?

Buffers! Photo by Nigel A. James

Alpha and Omega
Is this the end of the line? Or is it the begining of the line? It is both in one! It is "b" when going from "a" to "b", but, when returning to "a" it has become "a" and the original "a" has become "b". Coming is sometimes going and the other way around as well!