On this screen, you are defining your data model.
First, you need to define your main types. These are the types that would be re-usable. For example, in a school management app, you may have “pupil” as a main type. That way it can be linked to and reused in other main types such as classes, exams, homework, etc.
Sub-types contain anything that is related to a main type and typically does not get re-used outside of that main type. For example, “marks” can be contained in a table or a list of marks within the “pupil” main type. If you have another main type describing “class”, you will be able to link up the mark record to each class record.
In summary, to define your main types, select the main types of objects that you want to track in your application. It could be projects, cars, boats, pupils, club members, etc.