The terms TU1 limit and TU2 limit are not mentioned in the regulations but are commonly used (although some also refer to them as To limit, T1 limit, Tabs limit, and T2 limit).
The regulations refer to the maximum allowable error and how it can be calculated:
The TU1 limit can be calculated by subtracting the maximum allowable error from the nominal quantity, and the TU2 limit is determined by subtracting twice the maximum allowable error from the nominal quantity.
You may have come across "Class A" and "Class B" in European Directive 76/211/EEC, which specify different maximum allowable errors. In the 1976 version of the Directive, these classes were indeed present but were removed in 1978. The version with all the amendments made over the years is available at the end of the same website of the European Union ("All consolidated versions").