# Prescriptive vs Performance vs Proprietary Specifications

> Source: https://deviationcheck.com/guides/specification-types/

Before you can call a submittal a deviation, you have to know what kind of spec it answers to. The same cut sheet can be compliant against one type of spec and a clear miss against another. A [spec section](/glossary/spec-section/) is written using a handful of specifying methods, and the one in play decides how much room a sub has to pick a product.

## Prescriptive (descriptive) specifications

A [prescriptive specification](/glossary/prescriptive-specification/), also called descriptive, states exactly what to provide: the material, the dimensions, the construction, the method. It does not have to name a brand, but it describes the product so closely that only a narrow set of products qualify. Think "0.032 inch aluminum, clear anodized, 1 inch insulation, U-factor not used" rather than "a window that performs."

Reviewing against a prescriptive spec is matching the submittal to a description, line by line. A value that does not match is a deviation, even when the sub's product is "just as good." The spec already decided what good means.

## Performance specifications

A [performance specification](/glossary/performance-specification/) states the result the product has to achieve - a fire rating, a load, a U-factor, an STC rating - and leaves the means to the sub. Any product that hits the stated performance is acceptable, brand aside.

Reviewing a performance spec is about proof, not brand. Does the submittal document the required rating or capacity? The deviation here is usually a gap in the numbers, a missing or insufficient value, rather than the wrong manufacturer. That is one of the [six deviation categories](/guides/submittal-deviation-categories/) a review sorts findings into.

## Proprietary specifications

A [proprietary specification](/glossary/proprietary-specification/) names a specific manufacturer and product. It comes in two forms, and the difference decides whether a sub has any room at all:

- **Open proprietary** names the product and adds "or equal" language, leaving a door for an [or-equal](/glossary/or-equal/) submission as long as the sub proves equality.
- **Closed proprietary** names one product, no "or equal," and allows nothing else without a formal [substitution request](/glossary/substitution-request/) the design team approves.

The [basis of design](/glossary/basis-of-design/) is usually written as an open-proprietary entry: one named product the rest of the design was sized around, with equals allowed on proof.

## Reference standard, the fourth method

CSI formally recognizes a fourth method: reference-standard specifying, which requires a product to comply with a published standard such as an ASTM, ANSI, or UL standard, without describing the product itself. "Provide" plus the standard number does the work. Most real spec sections mix methods, a prescriptive Part 2 with performance requirements and reference standards woven through, so a single section can hold more than one at once.

## The differences at a glance

| Type | What it pins down | The sub's freedom | Substitution path |
|---|---|---|---|
| Prescriptive (descriptive) | The product description and method | Low - match the description | Substitution request |
| Performance | The result to achieve | High - any product that meets it | None needed; prove the result |
| Proprietary, open | One named product, or equal | Low - the named product or a proven equal | Or-equal submission |
| Proprietary, closed | One named product, no equal | None | Formal substitution request only |
| Reference standard | Compliance with a cited standard | Any product meeting the standard | Prove the standard |

## What this means when you review a submittal

Identify the method before you judge the package. A sub who swaps a product under a closed proprietary spec has a deviation no matter how good the product is. A sub who meets the number under a performance spec is compliant even with a brand the reviewer has never heard of. Most of the manufacturer-substitution deviations a review flags trace back to a sub treating a prescriptive or proprietary spec as if it were performance.

## Common mistakes

- **Reading a prescriptive spec as performance.** "It performs the same" does not matter when the spec described an exact product. Match the description.
- **Treating an open proprietary "or equal" as a blank check.** The sub still has to prove the equal meets the named product's salient characteristics. See [or-equal vs substitution request](/guides/or-equal-vs-substitution/).
- **Missing the reference standard.** A product can match the description and still fail the ASTM or UL standard the same section cites. Both have to hold.
- **Assuming performance means no paperwork.** Performance is the easiest method to meet and the easiest to under-document. The rating still has to be shown, not asserted.

When you are not sure which way a spec cuts, [run a review](/#pricing) and see the deviations flagged against the actual spec language. For where these methods live inside a section, see [how to read a spec section](/guides/how-to-read-a-spec-section/).

## Frequently asked questions

### What is a prescriptive specification?

A prescriptive specification, also called descriptive, states exactly what to provide - the materials, dimensions, and methods - without naming a manufacturer. The sub has to match what is described, which leaves little room to substitute.

### What is the difference between a prescriptive and a performance specification?

A prescriptive spec defines the means: the exact product, material, or method. A performance spec defines the end: the rating, capacity, or result the product must achieve, and lets the sub choose any product that meets it. Prescriptive constrains the how; performance constrains the outcome.

### How many types of construction specifications are there?

CSI describes four methods of specifying: descriptive, performance, reference standard, and proprietary. In everyday use, prescriptive is the umbrella term for descriptive and proprietary specifying - the methods that name what to provide - in contrast to performance specifying.

### Does a proprietary specification allow substitutions?

Only if it is written as an open proprietary spec, which names a product and adds or-equal language. A closed proprietary spec names one product with no or-equal option and allows no substitution unless the design team approves a formal substitution request.