The interaction volume is the volume of the sample that the incoming electrons are interacting with. Each electron will penetrate to a different depth, and as it penetrates the material it will take a different path. The paths can be modelled using a Monte Carlo technique. Shown here are examples of the output from such calculations.
The energy remaining to the electron at each depth determines the type of interaction. So that schematically we can represent the interaction volume according to the image below.