"Official" ISO rim width is the inside width. The outside width for a 21 mm ISO width is on the order of 25 mm, or so. Not all rims have the same difference between outside and inside width. An Alex rim (Adventurer, I think) recently discussed in another thread was 25mm out and 19 mm in while the CR18 is 19 and 22.5.

I'm not looking at the Schwalbe catalog, but my recollection is that it refers to ISO width.