The "as detailed' tolerance is the tolerance the designer can incorporate into his design. In this case the designer is afforded the latitude of increasing the root opening by 2 mm. The designer would have to indicate the required root opening in the welding symbol.
The as "fit-up tolerance" is added to the dimensions indicated by the welding symbol. If the welding symbols specifies the root opening of "X", it could be as small as "X-t" or as large as "X+t", where t is the as fit-up tolerance.
Best regards -Al