I use a custom save button, but now I must also create a custom cancel button. What is the default microflow for cancel behaviour?
Probably in your case a rollback object activity would be sufficient in your customized cancel microflow.
If the case is more advanced, it would highly depend on the actions you would like to simulate in that microflow whether you have to use rollback or delete in your custom cancel microflow action.
If this microflow only creates the object and doesn't commit the object, it would be sufficient to use a rollback. Since the release of the 2.4.4 version of the Mendix Business Modeler, a default action is available "object / rollback".
If it commits the object, you should remove the object instead of just calling a rollback. Rollback will not remove the object because it isn't 'new' anymore after the first commit.