Role of Platform Manager and System Management Hub

What is the role of Platform Manager and System Management Hub ?
What the are components/products controlled/dependant on Platform Manager and System Management Hub ?

Platform Manager controls MWS, IS, Broker depending on the installations for display in Command Central.

For System Management Hub I am noct quite sure, but for my current installations there was no System Management Hub required.
Most likely it will be needed for CentraSite-Installations, which we are not using currently,

Regards,
Holger