When configuring an enrollment source, there is a link to documentation for the Org Identity Mode.

Currently, this link points to: 

https://spaces.at.internet2.edu/display/COmanage/Registry+Enrollment+Flow+Configuration#RegistryEnrollmentFlowConfiguration-OrganizationalIdentitySources

 

though, this resource doesn't really talk about the modes. Suggest pointing to this resource instead that talks specifically about Enrollment Source Modes:

https://spaces.at.internet2.edu/display/COmanage/Enrollment+Sources