AutoCAD Electrical puts the tag name of the component on this attribute, names like "PB101" or "CR-55" (24 characters maximum). The default value you assign to this attribute definition at the library symbol level (that is, ".dwg" file of the symbol opened and displayed in AutoCAD) becomes the family code character string AutoCAD Electrical uses to build the tag name of the component when the block is inserted into your wiring diagram. This default value character string is used as the Family Code (%F) portion of the tag format code of the drawing you set up in the Drawing Properties 
Components dialog box (example %F%N).
For example, use "PB" for a default value of the attribute definition TAG1 if the family name you want AutoCAD Electrical to use is "PB" (examples: "PB100", "100-PB", "PB4-100"). You can override this family name at component insertion time, a later edit, or automatically by use of the "wd_fam.dat " mapping file.
If the TAG1 attribute carries no default value then AutoCAD Electrical uses the FAMILY attribute value of that symbol.