Hi Brian, see if this helps:

The most basic way of using extrude (which I think you are already familiar with), is to select a curve like this:



Which then after extrusion will result in an object like this:



However, if the curve is not a flat planar curve, it won't become automatically capped at the top and bottom when doing the extrude.

There is another way that you can use extrude instead of using a curve as the initial object. It is also supported to use a surface as the initial object, like this:



After extruding, with that surface selected as the input, this object will be created:




That type of extrusion will always have a cap even if the starting surface was not planar - this is because MoI is able to make a copy of the starting surface to the other end to construct the cap in this case.

Does that make sense?

- Michael