This is a rough template for something like a technical specification. There are so many different levels this can be done at that I wanted to give a framework with emphasis on the parts that we don't do well, that are poorly understood, and that are challenging. There is room for all of the rest, and this is only a template.
You will note that there is less specific emphasis on exactly how solutions should be laid out when the technical area arrives, because that is highly dependent on the problem and solution. One of the most serious drawbacks of government-style requirements engineering specifications is that the format tends to overwhelm form and get in the way of communicating the actual information.
At the same time, our current ad-hoc process is completely insufficient - "here are some states we have or some message types that will be sent" is so low-information that it is unactionable.
Having done systems engineering level formal specifications for government contractors, I want to avoid the process-for-the-sake-of-process rigidity of those specifications (because they are time consuming and often impede results) and recognise the limited nature of our resources to make the best use of them.
This is a tool. Extended discussions about formal requirements language are out-of-scope here; if you plan to write a spec, you do what you need to do. This reflects a superset of what I, as a team lead and developer, would expect to see and be able to work with.
Motivation for what you are proposing. If you feed the specification consumer a plan and they don't know why you're doing it, it's not really a plan for anything.
We're all technical, and tend to think the technical parts showing the bits and pieces of our solution are the most important thing, but this section is actually the cornerstone of your document, and it will be the hardest to write.
It is not sufficient, but it is absolutely necessary.
This section should contain at least the following:
Context for the problem what you are specifying will solve. You must situate what you are proposing.
Summary of the problem.
This should contain information such as the origin of the problem and why it is worth solving, who the problem impacts, how it impacts them, and how the problem interacts with and impacts other parts of the system.
Discussion of how this problem may currently be being handled, or, if it is not, why the current system cannot handle it, and the pros and cons of leaving things as they are now
This is very high level. What, if you had five minutes to explain the solution to someone who does not know all the details of the system, would you say to someone to give them an idea of what the solution does or involves. Presume an ill-informed audience who is not going to pay attention to the details YOU care about - that comes later.
This is important to be able to frame who needs the document as well as guiding you, as the writer, to the perspectives you need to present in the solution
This needs to be described in terms of each part of the system it impacts.
This should include the following, where applicable, and in an order that makes sense:
This should make it clear what needs to be incorporated into the solution so that service can give appropriate feedback. This includes tools, logging, and other infrastructure that may need to be developed or modified.