A performance requirement differs from a prescriptive requirement. A performance requirement states objective standards to be achieved and describes methods that can be used to demonstrate compliance. A prescriptive requirement specifies, in detail, materials, methods and design methodology to be used in such a way that, following the prescription, presumes that the standard is met, often without stating the particular standard. 

 

Example - you hire a contractor to install an HVAC system in your house under a performance based contract and the standard is "72^oF all year round in the house". In this case, the contractor is solely responsible for the means, methods, materials and layout. The end result is that it is his responsibility to ensure that your house can remain at 72^oF year round. Conversely, you hire a contractor and provide him a detailed set of plans and specifications with direction on methods, materials, sizing, and layout. His only duty is to build the system as specified. He is not responsible for the comfort level in the home - only that the system complies 100% with the plans and specification. This is a prescriptive based contract.

 

The same principals hold for a regulation. If a prescriptive regulation states that a design shall use purple pipe - then you have to use purple pipe. If it works – great! But, if it doesn't work, you still had a duty to specify what was required by the regulation.   If a performance regulation states that you shall use a pipe of sufficient structural integrity to convey the wastewater - then you are free to specify any color pipe. But, you as the designer, are now responsible for the performance of that pipe. 

 

A prescriptive regulation is simply a recipe. Overall - there is greater risk to the designer with a performance based regulation. That is why the two can't be mixed - because from a liability standpoint, you are either following the prescription or you are responsible for its performance. It's either-or, but not both.