Bill of materials (BOM)

The three lists are cousins with different jobs, and the differences are worth exact words. A BOM belongs to an artefact: this isometric, this skid, this fabrication drawing, listing every component the assembly consumes — its scope is the drawing's scope, its consumer the fabricator. An MTO belongs to the project: quantities extracted and aggregated across many drawings and the model, structured for procurement and estimating — buy this much pipe, of these classes, by this date. A BOQ belongs to the contract: measured items of work, labour included, structured under measurement rules for tendering and payment. Same underlying plant; three structures, three audiences, three standards of care.

The mappings between them are where quantities go to disagree. Isometric BOMs sum to the piping MTO — minus the allowances, plus the growth. The MTO informs the BOQ's material content — restructured under measurement rules that count differently. Reconciling the three is routine, manual and never quite finished; and a conversation in which two parties say "the bill" meaning different lists is a conversation heading somewhere expensive.