[1] Digi-CA™ is the Certificate Engine core software that includes the interfaces, modules and applications [IP] that are common to both the Digi-CA Server™ software and the Digi-CA™ Service.
The importance of this common Intellectual Property [IP] is that the Service and the Server versions of the Digi-CA™ [2] are effectively interchangeable.
Therefore, starting with one version of Digi-CA™ and migrating to the other as your requirements grow will not require a complete system replacement (as is the case with Traditional CA [3]s) and thereby saves you considerable costs, should such a requirement arise.
Links:
[1] https://www.digi-sign.com/downloads/download.php?id=digi-ca-pdf
[2] https://www.digi-sign.com/digi-ca
[3] https://www.digi-sign.com/certificate+authority/traditional+ca