SPS is the software that sends all of your new objects, changes to attributes and password resets to us so that our system can process them. 

As of version 4, SPS no longer needs to be on every Domain Controller. It will pick up any changes as they sync across to the one it is on. In certain situations we may need to install it on each Domain Controller depending on your setup, however we will identify this during on-boarding or if you inform us you are making major changes to your setup.