In the overview of all the apps, you find an app called "clients and staff". If you open this app, you will see an overview of your clients. The clients you see here, are all the clients in your system. You can use the filters at the top to see a specific part of your members. In this app, you can attach yourself or others coaches in this system to the clients. You do this in the client's profile under the tab "coaching".
Once you've attached yourself as a coach to clients, you will be able to see those clients in the coaching app. From there, you can choose all sorts of coaching options for the clients, such as "create new trainingplan" or "set new goal".
So the difference between the two clients lists is this: In the Clients and Staff app, you find an overview of all the clients in your system. In the Coaching app, you only see the clients that you specifically are connected to as a coach. So if you don't want the coaches in your system to be able to see clients that are not assigned to them, you can give them the privilege "coach" and leave out everything else. This makes sure they will only see what you want them to see; the clients assigned to them.