While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation (e.g. for vibrations caused by industrial applications).
Field fabricated is typically made from 16 gauge carbon steel, all welded, per local codes, which is then externally treated with fireproofing.
Fireproofing was an important selling point because of the fire that had devastated nearby Chinatown the previous year.