There are many attributes for each field type. By using the appropriate standard field, the attributes are applied.
Adding a field that is not on the Standard Field Guide list creates a custom field. For custom fields, you choose the ID, field type, label, and so on.
If you use a standard field ID, but not the corresponding field type, the field is considered a custom field. For example, using picklist instead of enum for Department makes it a custom field.
To make the custom field reportable, once the field is defined in JRDM XML field has to be added in Provisioning, Configure Reportable Custom Fields and data has to be synchronize.