Program specification is something that I am familiar with at my job! When a designer gives the customer a program specification he or she is giving them an idea of what the program will do, and the customer has to agree, it's somewhat of a contract between the two parties. This program specification will allow the customer to set up his to her on guidelines on how they want to use the program. For example, when the information systems department wants to install a program on the diet office computers, we must come to an agreement on how we want and need the program to operate.
This comes back to functionality, which is the major component in any specification. The purpose of this agreement is so that everyone involved in the use of the program will have confidence in the results of the program. In addition, since program specification is a form of contract, the way it's constructed can be based on the program specification by the customer.