The permissions tab of the importer screen allows you to control if records can be inserted, updated or deleted.
Import wp uses a unique identifier to determine if a record should be inserted / updated / or deleted, leaving this field empty will use the template defaults, otherwise you can select from the predefined dropdown list or type a custom value to be used.
Restrict which fields can be imported
This section allows you to limit what the importer can do either create / update / or delete, each can be refined further to allow or deny a specific set of fields / properties.
Clicking on a checkbox will expand any extra options, and allow you to save and continue onto the final step.
The import dropdown allows you to choose what restrictions will be applied per import method.
The all fields option means there are no restrictions for that import method.
Only the following fields
Only the following fields allows you to specify a list of fields that can be imported, enter each field or field pattern on a new line.
None of the following fields
None of the following fields allows you to specify a list of fields that will not be imported, enter each field or field pattern on a new line.