Custom fields are specifics fields you can add to a list in addition to those which already exist. update langs set en=email address, Firstname, Lastname, etc…) They are useful to complete the subscriber’s profile.
You can also use it in this cases:
- To segment your list: the custom field can be uses as conditions to make filters
- Triggers: For automatic updates in the subscriber’s profile
- Forms: to create fields you can display in a subscription’s form, for example