In the CDS, fields define individual data items that can be used to store data in an entity.
You can use the Maker portal to edit system fields and to create, edit, or delete custom fields.
With other systems, a developer will first create a column in the database table, then add the attribute to the data access layer, then add the attribute to the business object layer, and then to the user interface layer. In the CDS, it is different; you create a field, and the Power Platform adds a column in the database table, the attribute into the data access layer, and the field into the business object layer, all automatically. When added to a form, a field becomes a control. When added to a view, a field is a column.
This power makes it very quick to build applications on the CDS. But with this power comes a compromise; with the CDS, your data layer is your business layer, and your business layer is your user interface layer. You cannot create new data types. You cannot combine fields...
