P&ID (Piping & Instrument Diagram) is a basic engineering document that indicates process requirements, equipment, piping, instrumentation, and logic relations in a plant. It should show all lines, flows, equipment, piping components, instrumentation for control, monitoring, and shutdown, and battery limits. P&IDs are used for detail engineering, planning, construction, commissioning, operation, and maintenance. They go through a verification process and are updated as built. Standard symbols are used to represent different items on P&IDs.