By "safetran signals" with "Restricting State = On or Off" I presume you mean the JointedRail signals by NS37, which are custom scripted signals, which is why you won't find anything in the manual about them.
It does as it says - it allows the signal to show a restricting state, but I've never figured out the exact scenarios under which this happens, whether it is shown when the track ahead is unsignalled, or if it drops to a restricting state (instead of an advance) when the next signal is red, or some other case.