What is the difference between a Performance Specification, Systems-Sub Systems Specification and a Requirements Traceability Matrix and which one should be used for a program that is moving towards Milestone A?
A system approaching a Milestone A lies presumably on the Major Capability Acquisition Pathway (https://aaf.dau.edu/aaf/mca/) and your top level policy document is DODI 5000.85 (https://www.esd.whs.mil/Portals/54/Documents/DD/issuances/dodi/500085p.pdf). Accordingly, your system should have a draft Capability Development Document (CDD) at this time. If so, your System Level Performance Specification should also be a draft. You should NOT lock in your system level performance specification until System Functional Review (SFR) or Preliminary Design Review (PDR) sometime after MS A and CDD approval/validation. If you are on another acquisition pathway (https://aaf.dau.edu/) with another warfighting requirement besides a draft CDD the logic still applies, but the specific details might be different.
Assuming your program has a draft CDD and is on the MCA pathway, the Feb 2022 OSD SE Guidebook (https://ac.cto.mil/wp-content/uploads/2022/02/Systems-Eng-Guidebook_Feb2022-Cleared-slp.pdfsays) discusses SFR in chapter 3.3 and PDR in chapter 3.4.
Now, let’s define what a specification is. According to MIL-STD-961E (https://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=36063defines) a specification is defined as follows:
“3.43 Specification. A document prepared to support acquisition that describes essential technical requirements for materiel and the criteria for determining whether those requirements are met.”
You asked the difference between a performance spec and a system/subsystem spec. You really ought to be asking two slightly different questions. 1) What is the difference between a performance spec and a detailed spec? 2) What is the difference between a system spec and a sub-system spec? System and sub-system specs can be either performance or detailed, but in DoD contracting, we generally prefer performance specs.
MIL-STD-961E defines a PERFORMANCE specification as follows:
“3.35 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. Both defense specifications and program-unique specifications may be designated as a performance specification.”
And a detailed specification is defined as:
“3.13 DETAIL SPECIFICATION. A specification that specifies design requirements, such as materials to be used, how a requirement is to be achieved, or how an item is to be fabricated or constructed. A specification that contains both performance and detail requirements is still considered a detail specification. Both defense specifications and program-unique specifications may be designated as a detail specification.”
Now let’s define system versus subsystem (or “item” as you will see) specifications. Again, per MIL-STD-961E:
"3.25 Item specification. A type of program-unique specification that describes the form, fit, and function and method for acceptance of parts, components, and other items that are elements of a system." (This is for your subsystems, components and parts).
"3.47 System specification. A type of program-unique specification that describes the requirements and verification of the requirements for a combination of elements that must function together to produce the capabilities required to fulfill a mission need, including hardware, equipment, software, or any combination thereof." (This will usually be your top level specification for the entire, fully integrated and supported product.)
For guidance on how to identify subsystems within a system, we recommend the Work Breakdown Structure (WBS) approach per MIL-STD-881F available at: https://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=36026.
A traceability matrix is simply a tool for tracing the lower level (parts, components, subsystem, system specs) up to the CDD and vice versa. The idea is simply from the top down view we want all the valid requirements in the CDD flowing down to the hundreds or thousands of specific requirements in the lower level specs. And from the bottom-up view, we expect that every specific requirement for something as simple as a circuit card is traceable all the way up to a warfighting requirement in the CDD. If you want to read a bit more about traceability we recommend Para 4.1.4, Requirements Management Process in the February 2022 OSD SE Guidebook (https://ac.cto.mil/wp-content/uploads/2022/02/Systems-Eng-Guidebook_Feb2022-Cleared-slp.pdf).
And your final question was “which should be used at MS A?” Unless you have conducted your SFR as mentioned above, the appropriate specification at this time is almost certainly going to be a DRAFT System Specification, probably expressed as a performance specification.