In your inheritance split you need to define an object that’s a generalization. The outgoing flows are for the specializations that can appear. The next action should be a cast action that returns the specialization.
There's learning paths in the mendix academy that explain these types of actions in more detail. Inheritance is part of the following module: https://gettingstarted.mendixcloud.com/link/module/50/lecture/385
I'd recommend going through it to understand more about inheritance and generalisations