Correct! When two or more faces are selected that are facing in different directions.
However,
When one face is selected, E will extrude in the normal direction (local Z axis).
When two or more faces are selected E will extrude in the median direction of the Normals of all the selected faces.
Unless, when selecting the four faces of a cube in a loop for example, i.e. Top, Bottom, Left and Right or Left, Right, Front and Back, then the extrude will happen in the Global Z axis.
But you can also select the axis by pressing X,Y or Z.
