Activity-on-Node and Activity-on-Arrow diagrams are two different ways of showing a project schedule network.
With Activity-on-Arrow or AOA schedule diagrams, the activity occurs along the arrow. As a result, you are constrained to showing only finish-to-start relationships. As a result, dummy nodes had to be added in order to show more complex relationships.
If you place the activities on the nodes as with Activity-on-Node (AON) diagrams, now the arrows can go in any direction and you can represent the full range of dependencies: start-to-finish, finish-to-start, start-to-start, and finish-to-finish.
In general, because of the serious constraint, Activity-on-Arrow diagrams are now rarely used.