Dear Rivera Adriana
The situation is like this!
There are two possibilities to Reverse transaction and flows:
1. Reverse trough FTR_EDIT transaction -> "Reverse" button
From here you can reverse Activity. Activity depends on Processing Category (like Order - Contract - Settlement, etc).
If you have Settlement - it is very usefull. Becasue when you do reverse operation you reverse Settlement activity to Contract activity + First flow (or all flows if they were not reversed with the second possibility). When you've go Contract activity - you can still work with transaction - to enter new conditions, etc.
When you don't have Settlement activity, you reverse Contract activity. Before Contract activity there is no any other activity (usually) - so the transaction is fully reversed. You won't be able to work with transaction anymore.
2. Reverse mode at "Cash flow" tab.
Here you can reverse flows except the first flow.