Hint: IEC61346-1 was based upon the DIN standard from Germany. This table is based upon IEC61346-2, which made significant changes to the letter assignments from IEC61346-1, but includes an optional 2-letter table. The IEC60617 library symbols are metric sized with 4mm text height and the tag letters are from the optional 2-letter table in IEC81346-2. class designation letters) predominantly from the IEC61346-1 standard, mostly 1-letter tags that denote the circuit function of a device. IEC4 blocks are metric sized with 4mm text height. IEC2 blocks are metric sized with 2mm text height. Below is an excerpt from some of my training material that explains the differences between the IEC libraries supplied with AutoCAD Electrical: