It is also called a single-line diagram. It shows the utility service, switchgear, transformers, panels, feeders and large equipment. It is not to scale.
An electrical estimator reads feeder sizes, breaker sizes and equipment ratings from it. The panel schedules then give the branch circuits.
List each piece of gear with its rating. Send that list to your supplier for a quote. Measure feeder lengths from the floor plans, not from the diagram.
Compare each panel name on the diagram with the panel schedules and the plans. Names that do not match need an RFI.