If you are editing a form that has already been assigned to the page and you removed or missed any Fields (i.e. Username, Password) from the Form, you will get the error message that will display the Form is used on another page (i.e. Registration page) and it will require to add the missing fields (i.e., Username, Password)
So if the same form needs to be used on different pages but having different Fields, this conflicting message will appear during the editing and designing of the Form.