Per this link:
https://docs.microsoft.com/en-us/azure/iot-dps/concepts-service
Under Individual enrollment:
The registration ID in an individual enrollment is alphanumeric, lowercase, and may contain hyphens.
Under Registration ID:
The registration ID is alphanumeric, case insensitive, and may contain special characters including colon, period, underscore and hyphen.
Why is one more restrictive than the other?

