The terminology is genuinely confusing. Three documents — Mill Test Certificate, Certificate of Conformance, and Certificate of Compliance — all get called "the cert" in day-to-day operations, all get attached to steel shipments, and all look similar enough that the differences are easy to miss until an auditor asks a specific question and you realize you've been sending the wrong one.

The confusion causes real problems: audit findings, customer rejections, and supply chain disputes that take weeks to resolve. Getting the terminology straight is a prerequisite for getting the document control process right.

Mill Test Certificate (MTC) / Test Report

The MTC is issued by the steel mill and documents the actual test results for a specific heat of material.

It contains: the heat number that identifies the melt, the chemical composition determined by ladle analysis, the mechanical test results (yield strength, tensile strength, elongation, reduction of area — at minimum), the applicable standard and revision, the product form, and the identity and signature of the authorized inspector.

The authority behind an MTC comes from who performed and witnessed the testing. Under EN 10204:

Type 3.1 — inspection by the mill's own authorized inspector (a person specifically designated for this function within the mill's quality system)
Type 3.2 — inspection witnessed by an independent third-party inspection body (TÜV, Lloyds, Bureau Veritas, etc.)

When traceability to specific tested material is required — pressure vessels, structural applications, pipelines, critical weldments — the MTC is the primary document. It is not a statement of conformance; it is a factual record of what was tested and what the results were.

Certificate of Conformance (CoC)

A CoC is issued by the supplier, which may or may not be the mill. A distributor, service center, or fabricator issues a CoC; the mill issues an MTC (or test report under ASTM terminology).

The CoC is a statement: "This material conforms to the requirements of [standard] and [customer's purchase order]." It is the direct supplier's declaration, not the mill's test record. It may include test data, or it may reference the mill cert by heat number without reproducing the data.

When a CoC is appropriate:

The customer wants a statement from their direct supplier (not just a relay of the mill's document) that the material meets their specific PO requirements
The transaction involves multiple supply chain steps and the customer wants each party in the chain to take accountability for conformance
The application standard requires a CoC in addition to (not instead of) the MTC

The critical distinction: A CoC can reference an MTC, but it cannot replace an MTC when heat-specific test data is required. If an auditor asks for the mill cert and you provide a CoC that says "we confirm this material is A572-50," you have not provided the test data. The auditor will ask again.

A CoC can be self-certified (signed by the supplier's quality representative) or witnessed by a third party, depending on the application and customer requirements.

Certificate of Compliance

Certificate of Compliance is a term used across industries with slightly different meanings depending on the context. In many commercial settings, it is used interchangeably with Certificate of Conformance — a statement from the supplier that requirements have been met.

In regulated industries — particularly aerospace (AS9100), defense, and nuclear applications — Certificate of Compliance has a more specific meaning. It is a formal declaration that all requirements of a contract, specification, or purchase order have been met, and it may have mandatory content requirements defined by the applicable quality standard. Under AS9100 and NADCAP, a Certificate of Compliance often must include specific traceability information, conformance language, and authorized signatory requirements.

The practical guidance: if your customer uses the term "Certificate of Compliance," ask them to specify what document content they require. Do not assume it means the same as a CoC. In regulated industries, the difference is meaningful.

The Auditor Question

When an auditor conducting a supplier quality audit, a code inspection, or a customer quality review asks for "the cert," they are almost always asking for the MTC — the original document from the mill with heat-specific test data.

A CoC that references the MTC heat number is acceptable in many contexts as a supplementary document demonstrating the supply chain's conformance statement. It is not acceptable as a replacement for the MTC when the auditor needs to verify the actual test results.

The Common Mix-Up and Its Consequences

Here is the scenario that plays out regularly in metals distribution:

A distributor supplies plate to a fabricator. The distributor issues its own branded CoC to the fabricator, referencing the mill cert heat number. The fabricator delivers the finished assembly to the end customer. The customer's quality team requests the material documentation during incoming inspection. The fabricator provides the distributor's CoC.

The customer's auditor asks for the underlying mill cert — the original document from the steel producer with the heat analysis and mechanical test results. The fabricator goes back to the distributor. The distributor goes to their receiving records. The mill cert is not on file — it was not retained when the material was originally purchased from the steel producer.

The audit finding: incomplete material documentation. The remediation: obtain the original cert from the mill (possible if the mill retains records; not always possible for older heats). The downstream effect: shipment hold, delayed project completion, customer confidence damage.

The fix is straightforward: distributors and service centers must retain the original mill cert — not just issue their own CoC. The CoC can sit alongside the MTC; it cannot replace it.

What Each Document Requires From Your Suppliers

When writing purchase order requirements, specify what you need:

"MTC per EN 10204 Type 3.1" — original mill cert, mill-authorized inspector
"MTC per EN 10204 Type 3.2" — original mill cert, third-party witnessed
"Test Report per ASTM A6/A6M" — ASTM-standard mill test report for structural shapes
"Supplier Certificate of Conformance referencing original mill cert heat number" — CoC in addition to the MTC, not instead of it

The specificity eliminates the ambiguity that produces the wrong document showing up at the wrong time.

MTC vs Certificate of Conformance vs Certificate of Compliance: Only One Satisfies Your Auditor.