In the self-registration configuration is must be possible to define the organisations where the new registered users are added. By default (no config), the default organisation is used. However, it could be usefull to add self-registered users to the default organisation and a organisation called "external users" to separate the users and the learning offering from the "internal" users that are in the company org unit.
The field is mandatory and must contain something. If the configured value is not valid (e.g. deleted), the default is used.