Create a "global" value for multiple settings Is it possible to create a single variable or defined value that can be used in multiple places -- and even in multiple ways? 

Frame example:

** I would like to identify frame 250 as such a value. Some materials would, on that frame, have a mix shader move transparency to zero, and other mix shaders would move to 1.0 at that point. And then, if needed by the flow of the animation, I could change the "250" value to 300, with the resulting changes reflected in all places it is use.

** The same value would identify the point at which an object begins or ends a rotation or movement... or the point at which a light is turned on or off....

Value Example:

** I would like to define a value (either 0 to 1, a percentage, a numeric range or similar) and then animate that single value to drive nodes that accept a similar input. A single animated value/variable could control multiple ObjectID inputs, for example, and a 0 to 1 range could drive transparency and Film exposure.

Anyway: Can I make a "magic value box"? The specific use case I have in mind right now is responding as timeline changes are made -- causing the need to re-time events in lots of temporally related but operationally different areas: motion, value, VSE strip placement, etc.

All input welcome.

thanks
rg
Well, you need to know about Drivers.

Drivers are scripts which main purpose is to control properties with other properties. In example the rotation of one object is controlled with the location of another object.

Drivers can use properties, numbers, transformations, and scripts, to control the values of properties.

Using a F-Curve, the driver reads the value of the Driver Value and sets the value of the selected property it was added to.
So for example, if the Driver Value is 2.0 the property can be 0.5.

https://docs.blender.org/manual/ru/dev/animation/drivers/introduction.html