Three systems, one common confusion
Ask five people on a project team to classify a metal stud partition and you will get three answers, each correct under a different standard. The estimator files it under an interior element, the specifier writes it as a work result in Division 09, and the model manager tags it with an OmniClass number that maps to neither.
None of them is wrong. They are answering different questions, and the friction starts when someone assumes there is one right answer.
MasterFormat answers "what work gets bought"
MasterFormat organizes construction by work result: the finished product of a trade's labor and materials. Its current structure runs 50 divisions, numbered 00 through 49, grouped into subgroups covering procurement and contracting requirements, general requirements, facility construction, facility services, site and infrastructure, and process equipment.
The numbering is six digits in three pairs. Division 09 covers finishes, section 09 21 16 covers gypsum board assemblies, and a specifier writing that section knows every bidder will find it in the same place.
Use MasterFormat when the question is contractual. A specification section maps to a subcontract, a subcontract maps to a bid package, and a bid package maps to a schedule of values line. That chain is the reason MasterFormat survived the arrival of every modeling tool that promised to replace it.
Where it fails you is early. At schematic design there is no gypsum board assembly to specify, because the team has not decided whether the partition is metal stud, CMU, or a demountable system, and MasterFormat has no place to put a decision that has not been made.
UniFormat answers "what part of the building is this"
UniFormat classifies by element: a physical part of the building defined by its function rather than by the trade that installs it. Exterior walls stay exterior walls whether they arrive as precast, curtain wall, or brick veneer.
The element groups run A through G, covering substructure, shell, interiors, services, equipment and furnishings, special construction and demolition, and building sitework. ASTM E1557 publishes the closely related UNIFORMAT II standard for elemental classification, which is what most cost consultants actually cite.
This is the schematic and design development language. An elemental estimate lets a cost consultant price a building before the trades are known, benchmark it against a comparable project, and tell an architect that the envelope is running high without waiting for a specification to exist.
OmniClass answers "what is this thing, in a database"
OmniClass is a faceted system built on ISO 12006-2, and it does not compete with the other two so much as contain them. Fifteen tables classify separate facets of the built environment: construction entities by function, spaces, elements, work results, products, phases, services, disciplines, organizational roles, materials and properties among them.
Table 22 is essentially MasterFormat. Table 21 is essentially UniFormat. The design lets you say that a specific product, from a specific manufacturer, installed as a specific work result, forms part of a specific element, during a specific project phase, by a specific role.
That expressiveness costs something. OmniClass rewards a team that has already decided how it will use the tables and punishes one that adopts it because a client asked for structured data. A number no query ever reads is overhead.
How to choose
Match the system to the decision in front of you.
If you are writing a document a subcontractor will bid, use MasterFormat, because their estimator's cost history is filed that way and asking them to translate is asking for a contingency.
If you are pricing or benchmarking before the trades are set, use UniFormat, and keep using it through design development so the cost narrative stays continuous instead of restarting when the specification arrives.
If you are structuring model data or a product database that has to serve both, use OmniClass tables as the spine and carry MasterFormat and UniFormat as attributes on the same record.
The part that actually breaks
Most teams pick correctly at each stage. The break happens between stages, when the schematic elemental estimate hands off to a specification and no one owns the mapping.
An exterior wall element carrying a budget of a certain dollar amount per square foot becomes six specification sections across four divisions. Somebody rebuilds that relationship by hand in a spreadsheet, the design changes twice, and by construction documents the estimate and the specification describe different buildings. The variance shows up as a bid day surprise and gets attributed to the market.
Carrying the classification codes on the object itself, rather than in a mapping spreadsheet maintained beside it, is what closes that gap. When a wall type holds its UniFormat element code and its MasterFormat sections at the same time, the estimate and the specification cannot drift apart without someone editing the object, and an edit to the object is visible.
That is a modest technical requirement and a large cultural one, which is roughly the story of classification in this industry.