A number of additions, to fill in the holes:
A system in software is called a “software system” . Once the marketing and requirements phases are completed, a software system gets defined as a product by a product definition and architecture group. A Software System Architecture Specification and a Functional Specification are developed, which provide a full map of all the functional blocks of the software system, the major algorithms, and their interactions, inputs data flow, and output data flow, as well as how map of its functions, respectively how they are controlled, managed, monitored, etc... A major element part of these specifications can be the type of real, or virtual hardware platforms on which the software will run. Designing the software for each functional block may be assigned to smaller groups. Functional Block Software Architecture Specifications may be developed, along with Block Functional Specifications. Algorithms are further specified in minutia details, It is followed by a Software Development Specifications, which defines the software mechanisms to be employed for implementing the algorithms and desired functionality in software. In parallel, there is a Quality and Test Plan Specification as one, or as two different documents, each defining the steps and tools to achieve the quality and testing targets. The software development – writing the programs - is done based on the latter, with unit testing done in multiple stages, from unit to system testing. Important to note that the Functional Specifications are the basis of the User Documentation development, which end up carrying most of the information of those specs..
For an airplane, the process is many times more complex, as the process multiplies, and there are so many specific technologies involved besides software. The requirements are external and internal – yes, company internal groups, from research, to manufacturing participate at defining a new product. An airplane system definition and architecture specifications is very complex, containing a a map of the various functional blocks of the airplane, etc… It is further divided into subsystem definition and architecture specifications.
For the airplane command and control subsystem, there is an subsystem architecture specification that goes into the details of the subsystem, defining the algorithms,
For larger airplane manufactures is not unusual to have the product specification done in paralele with the product simulation. Super-computers, and special programming languages, and compilers are used for modeling, and simulation of the functional blocks of the airplane, so that software can be run and the functioning of the functional blocks can be tested, and verified, through modeling, and simulation, long before any hardware piece has been physically ready for such a thing. It’s amazing, mind boggling….
The modeling and simulation during product development can be done even just for software systems, if the software systems are very large, and if the hardware platforms are developed in parallel, or not available immediately.
This is believe it or not... a very brief summary....