The stage at which the supplier, in conjunction with the user, develops the Functional Specification, showing what functions are to be provided to meet the user's requirements, stating what resources will be required, and providing the agreed basis of the system design.
A Functional Design Specification (FDS) details the solution to be provided to meet the Users and Functional Requirements.
It should be approved by the User and should form the basis of the design for both Hardware and Software.
All the functions, operator interactions control and sequencing associated for the prcoess automation should be clearly detailed. This allows the User to confirm, before the system is developed that the proposed solution fully meets the requirements.
The FDS provides the basis of the design of the system and is used to verify and validate the system during the testing, ensuring all the required functions are present and that that they operate correctly.
The FDS should cover
Control Modules such as PID Loops, Indicators etc
HMI Graphic displays
Equipment Basic Control
The Inputs and Outputs of the systems with cards and channels assigned to them.
Much of the FDS may be references to standard system objects.
But beware, sometimes what are supposed to be standard turn out otherwise, and some 'standard modules' turn out to be not what you really wanted.