Hello Louis
As you know, APIC-EM is a network controller for SDN. APIC-DC is a separate component that is used within the context of a Data Center.
Unlike APIC-EM, APIC-DC focuses more on the deployment, management, and monitoring of network-based applications via Application Network Profiles (ANPs) and End Point Groups (EPGs). It also interfaces with applications such as OpenStack and Fabric Packet Flow management. Some additional information you may find useful is included below:
The truth is that there isn’t that much information on it for a couple of reasons. First, both APIC-EM and APIC-DC seem to have been incorporated into Cisco DNA Center. In addition, it seems that the terminology of APIC-DC and the description of this as something separate slowly evolved into a single entity. You can see this, especially in the following datasheet for APIC, which includes many features for datacenters.
I hope this has been helpful!
Laz