ACT mapping is technically association between automatable parameters and virtual (SONAR internal, global) controls. Control Surface plug-in does yet another mapping then, between these virtual controls and physical controls on your hardware.
For example, let say "Channel tools" "Mid Gain" is ACT mapped to "ACT Slider 3". Then some plug-in (ACT MIDI, Generic Surface, VS-xxx, APro, etc) maps "Encoder 8" on your device to "ACT Slider 3". In such case, your "Encoder 8" controls "Mid Gain" in "Channel tools" (when it has the focus).
That procedure is not plug-in specific. You can map any parameter, which appear in the list of "Automations" (click on "Automation Lanes" and try to add an automation for the VST to see the complete list). But you can not map parameters which are not exposed there (that means VST is not informing the DAW about them).
There was (most probably still are, I have not checked with Platinum yet) some bugs in SONAR which make the mapping difficult. On my site you can find workaround. As I have mentioned, the mapping is global and persistent. So it should be done only once.