The definition of your entity is made through Attributes that store part of the information that concerns the entity.
Examples of attributes are: name, address, zip code, and city.
To create an attribute of an entity:
When you create an attribute, OutSystems Platform automatically manages for you the update of all entity records stored in the database, i.e., the entity records are automatically updated with the new attribute which is set with the default value (of the attribute's data type). Learn more about Default Values in Database.
Mandatory attributes are automatically validated at the user interface level by the OutSystems Platform. However, in the database, mandatory attributes are created allowing null values thus, at the database level there's no validation for mandatory attributes.
When you delete an attribute, Service Studio is permissive and you can delete an attribute whether it is being used or not. The application module is not valid and you must fix the elements where the attribute is being used.
Attributes Properties | Attributes Tab | Entity Definition