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Modular Scaffold to me is any system designed with fitting at predetermined and manufactured lengths. I.e. Safway Systems, Q.E.S., Wedgelock, Cuplok, etc.. which under that definition also qualifies the 2 European Frame "System" you have mentioned. I think if we looked up the definition of both words they both apply to all of the product lines mentioned. I guess its kind of like clamps. Depending where you are at on the planet, or in which part of the country you are in the same piece may have many names. Confusing I know, but reality none the less.
Thanks. Based on that, frame panel scaffold with the various cross braces could be called modular. It would be something to be a fly on the wall where a group of scaffold builders from all over the country and world were discussing their trade.