Nothing to do much for me. I just test for the practical idea. A "Average Active Sessions" chart on EM(from picture). Each block of color on the chart will sort from Waited Classes catagories(Point D). CPU (green) -> CPU Wait (light green) -> ... Other (purple).
How to practice for this idea?
A. Read on Oracle Documents
B. Just practice for fun :)
I tested Blocking Session. So, I found Application(red) waited class (Point A). During Database blocking session time (red), I tesed INSERT and SELECT. I found CPU, CPU Wait, Scheduler, USER I/O, Application and Configuration waites classes(Point B). After INSERT and SELECT sessions finished, I still see Application(red) waited class(Point C). That mean database has still Blocking Sessions. Anyway, that's not important. I just make sure about the sort each block color (Point B).
*** If Database no waited events for some waited classes, which waited class won't appear on Active Sessions page. ***
For new DBA, when you see the graph. You should know this graph colors are sorted by Waited Classes catagories (Point D).
Example Point B: that doesn't mean! TOP waited class is either "Configuration" or "Other". From chart, You have to identify each waited class on the chart. It's simple on EM, just click on each block color of waited class.
On other charts, just read the category on left chart before.