Software Module Specifications provide the details of various the modules of overal system
Most Process Automation systems are implemented using DCS or PLC based systems
These generally have a collection of standard reusable modules that need to be configured and/or programmed, the modules specs should provide detail of this along with examples of code.
What sort of modules does a process automation system have?
Control Modules such as PID Loops, Indicators etc
HMI Graphic displays
Equipment Basic Control
Note that DCS and some PLC sytems provide extensive inherent functions to handle such things as scanning inputs and setting outputs, some fault detection, communications etc. An SMS does not really need to define these as they are part of the standard system.
The objects in A Requirements Model are very likely to be closely related to the Software Modules themselves. For example the model might define the Equipment Modules and these be implemented in Software Modules for each.