semantics = all rims have a INNER wall ....and an OUTER wall ...hence, 2 walls !
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'What most people mean when they somewhat casually refer to a rim as 'double wall' is actually a 'box modular' constructed rim ....'box modular' is the proper term
Search "box modular rim" and "double wall rim" and see what they get you.
Double wall appears to be the industry standard nomenclature and I know I'd get a blank stare if I walked in my LBS and asked for a box modular rim.