The problem with using the TAG or Component Attribute-3 is that if you have valves or other components with identical long descriptions and NB, but different tag values then Isogen will not seperate the tags out, but will accumulate the valves and put the tag of the first valve from the PCF.

What I would recommend is to add Custom Data, then map this to an Isogen Material Attribute. This will seperate out any non unique tags and anot accumulate. The caveat is that custom data does not support Schemas at this time.