You haven’t specified what type of work and project stage you’re at but I would assume that you’re suppose to write it as part of the instruction to Contractor on next project phase.
That kind of work is kinda typical activity when you are in pre-FEED to FEED or FEED to EPC stage. You would have to narrate the philosophy of equipment layout, the level of detail of your input data and outstanding issues that the Contractor need to address on next phase.
Typically, the plant layout designer in charge of plot plan development would know these details and would write it as part of Project Specification.
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