Autodesk® Revit® (Matt Dillon) AB4013, on AU Virtual.
It was a class targeted at users with Intermediate expertise.
I learned this:
The Left hand side of a Reference Plane is the 'positive' + side. The 'handedness' of the plane is derived from start and end points of plane (the start point of the reference plane is the first point you click when drawing a new reference plane).
When you set a particular workplane as active, and use the default positive extrusion depth, the extrusion will be created on the positive side of the plane.
I have attempted to display this effect in the image below:
I'm sure this has been described elsewhere, but it was a eye-opener for me!