As mentioned in the announcement of Athens-RC1, the IO Platform has moved from a numerical version scheme to an alphabetical one. This aligns it with the Spring Data and Spring Cloud projects.

2.0.x was the last generation that followed the numerical scheme. Athens is the generation after 2.0.x and Brussels is the generation after Athens. Athens-SR3 is the third service release of the Athens generation. Brussels-SR1 is the first service release of the Brussels generation.

Service releases only pick up new maintenance releases so they should be a drop-in replacement for an earlier service release in the same generation. Switching to a new generation means picking up new minor or major versions so may take a little more effort.

If you are new to the Platform, Brussels-SR1 is, at the time of writing, the right place to start.