This is where the detail is specified. This can include highly detailed aspects on the control and operation of the process.
Functional requirements define the detailed knowledge of how the plant must work. This include operational steps, operator actions, calculations, data manipulation and processing, and other specific functionality
As a specifier remember that this will grow well into the life of the plant.
And if you have an existing process that you are replicating or updating that there is a huge amount of this knowledge in the plants. Sometimes it is buried in the code that us running the plant.
Fault handling should be described
Process Functional Requirements include:
HMI Graphic displays
Control Modules such as PID Loops, Indicators etc