Is SOLIDWORKS Manage an extension or add-on to PDM?

Both systems are bi-directionally integrated by SOLIDWORKS so that they form a single system.
However, when you look at the scale of the included functionality, Manage accounts for the majority of the delivered functionality. So in reality, PDM is an extension or ‘module’ of Manage not the other way around. Once you adopt Manage it assumes control of PDM and integrates PDM data.