Upvote:3
A "flex preis" ticket is a flexible ticket, valid on any train that runs the route you bought the ticket for. So you can take any KΓΆln to Berlin service on the days your ticket is valid (see the top of the ticket).
Your reservation will be for a specific train, but you are not limited to taking that train. Just when you take another train you will not have a guaranteed seat. Your reservation should have the train number, the car number and the seat number on it.
Given that you have a flexible ticket you do not run the risk of being on the wrong train, as long as you are on the right train route.