Comprehensive Version
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MainPhysics is what make your physics work. It is can be accessed
with self.MainPhysics from everywhere.

You can do these things:

    These if possible should be replaced with Script.Activated, Script.Activate and, Script.Deactivate.
    self.MainPhysics.Exists(x): x is "self." the name of a variable which stores a Script.
            This checks if the script is doing PhysicsTick.

    self.MainPhysics.Add(x): x is "self." the name of a variable which stores a Script.
            This makes the script start doing PhysicsTick. Only do this if 
            self.MainPhysics.Exists(x) is False

    self.MainPhysics.Remove(x):  x is "self." the name of a variable which stores a Script.
            This makes the script stop doing PhysicsTick. Only do this if 
            self.MainPhysics.Exists(x) is True
    
You have these variables which are made by default:
    Interval: how long before next PhysicsTick.
    FailedTickFailsafe: How many errors does it accept before crashing (set to 0 to disable crashing).
    FailedTickCount: How many errors have occurred during PhysicsTick
    Running: (Read only) returns wether still running
    Counter: (Read only) returns how many times did PhysicsTick happen
    DeltaTime: (Read only) returns how much time has passed since last PhysicsTick

Concise Version
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MainPhysics handles the physics event loop(on a separate thread). 
It is can be accessed by the attribute self.MainPhysics in every script.

Deprecated: use Script.Activated, Script.Activate() and, Script.Deactivate() when possible instead of the methods below

These are its available methods:

    self.MainPhysics.Exists(x): x is of type RG_Script. Returns a boolean value which
            indicates if a script is subscribed or not to this event loop.

    self.MainPhysics.Add(x): x is of type RG_Script. Subscribes x to event loop.

    self.MainPhysics.Remove(x):  x is of type RG_Script. Unsubscribes x to event loop.
    
You have these variables which are made by default:
    Interval: interval between physics ticks in seconds.
    FailedTickFailsafe: How the amount of exceptions accepted (set to 0 to accept all exceptions).
    FailedTickCount: The amount of exceptions occurred during PhysicsTick.
    Running: (Read only) returns wether event loop has been broken
    Counter: (Read only) returns the number of times PhysicsTick was called globally
    DeltaTime: (Read only) returns the time elapsed in terms of MainPhysics.Interval since last PhysicsTick started.