MIL-STD-961, which defines DoD specification practices, has a simple philosphy, which can be summarized as follows:
"3.29 Performance Specification. A specification that states requirements in terms of the required results with criteria for verifying compliance, but without stating the methods for achieving the required results. A performance specification defines the functional requirements for the item, the environment in which it must operate, and interface and interchangeability characteristics."
The current approved DIDs, which may be used to create configuration baseline documents, and thereby define system requirements, (and which have MIL-STD-961 as their source document) are as follows:
You can use these DIDs in your solicitation, and can cite MIL-DTL-31000B and MIL-STD-961 in your Statement of Objectives (SOO) or Statement of Work (SOW), as these are approved for use by DoD. Keep in mind that if you do use these documents in your contract solicitation, it is a good practice to tailor (edit) them so as to only include essential requirements.
If the government team is going to be writing the performance specification for the solicitation (sometimes referred to as a "Technical Requirements Document), then it is critical that the document be "performance-oriented" in nature. That is, it should only contain essential performance requirements. Any detail type requirements, such as dimensions or weights, should be mission-related, and should be so stated. For example,
"The ABC system must be transportable by the standard commercial 747, 757 and 767 aircraft." Or, "The ABC System must be transportable by a standard commercial Class III truck/tractor-trailer."
It is also critical that all interface, interoperability, and other mission essential standard type information be cited. For example:
"Software must comply with the current version of the World Wide Web Consortium XML standard." Or, "The system must comply with current FCC standards."