The System Requirement Document (SRD) defines system level functional and performance requirements for a system. The SRD is derived from the Capability Development Document (CDD), Concept of Operations (CONOPS), system-level performance metrics, mission threads/use cases, and usage environment and is developed but by the program office. It should include a system-level description of all software elements required by the preferred system concept.
The SRD is developed during the Technology Maturation & Risk Reduction (TMRR) Phase but a draft SRD should be developed for the Analysis of Alternatives (AoA) in the Materiel Solutions Analysis (MSA) Phase. The SRD is used by the program office to provide more detailed requirements than what’s provided in the CDD and is normally included in a solicitation package. The SRD should be finalized prior to the contract award. 
The SRD should be developed with feedback and input from the industrial base. Once the SRD is placed on contract, the contractor will further develop the specification and develop their own, more detailed requirements document and System Specifications; sometimes called a Weapons Systems Specification (WSS).
Definition of 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. 
Step in Developing a System Requirements Document (SRD)
The steps in developing an SRD are the same as any other requirements documents.
- Follow Requirements Development Steps
- Follow MIL-STD-961E “Defense and Program-Unique Specifications Format and Content” when developing the SRD.
- A contractor may build an internal document for managing the traceability of their requirements. If they call this an SRD, you might want to call yours a system specification to avoid confusing the two. Any requirements in their document should be ultimately traceable back to your system performance specification.
- The SRD and WSS should all be traceable to the Initial Capabilities Document (ICD) and CDD.
Software Requirements Specification (SRS)
The term “System Requirements Document” is a phrase commonly used to describe a “Software Requirements Specification.” If your acquisition is exclusively for software, you may call yours a “Software Requirements Specification” or “System Requirements Document”.
Definition of Software Requirements Specification (SRS): A software requirements specification is a document that describes what the software will do and how it will be expected to perform. It also describes the functionality the product needs to fulfill all stakeholder’s needs.
AcqLinks and References:
-  Defense Acquisition Guidebook (DAG)
-  MIL-STD-961 “Defense and Program-Unique Specifications Format and Content” – 16 July 2020
- Template: System Requirements Document (SRD)