I found a Wikipedia article that expands on motivations for the design: Titulus_(fortification)

The description in the wiki article and the orientation of the diagrams on page 63 of the Keeley document convince me that the ~~more common arrangement was to place the detatched screening wall segment on the exterior of the curtain with an~~ interior version sometimes used either instead of or in addition to the exterior screen.

Edit: I think I read the pg63 diagram orientations wrong, so I think my point is mostly that it can be either inside or outside, and that this whole PDF is really cool.