A control valve is an assembly of several components, and each can have its own tag, although they are all part of the same loop.

FV-1001 (Flow Valve): This tag represents the entire valve assembly (valve body + actuator). Sometimes LCV (Level Control Valve) or PCV (Pressure Control Valve) is used, but ISA prefers using the manipulated variable, e.g., Flow.
FY-1001 (I/P Converter): The I/P (current-to-pneumatic) converter that takes the 4-20mA signal and converts it to an air pressure signal.
FP-1001 (Positioner): The device on the valve that ensures the valve stem position accurately matches the control signal. Modern smart positioners often have the I/P function built-in.
FSL-1001 / FSH-1001 (Limit Switches): Switches that indicate the valve's closed (Low) or open (High) position.
FY-1001B (Solenoid Valve): A solenoid valve in the air line, often used for an emergency shutdown trip.

In modern systems, the main tag FV-1001 often represents the entire assembly, with the components listed on its datasheet.

Instrument Index creation and numbering conventions – Top 50 Question and Answers

Design Phase: Engineers populate the index based on P&IDs. It's used to generate datasheets, I/O lists, and procurement specifications.
Procurement Phase: The purchasing department uses the index to track orders, vendor details, and delivery status for all instruments.
Construction Phase: Installation teams use it to verify they have the correct instrument for the correct location and service.
Commissioning Phase: Technicians use it to plan and document loop checks, calibration, and functional tests.
Operations & Maintenance: The 'as-built' index is a vital tool for technicians to find instrument details, calibration history, and maintenance records.

Instrument Indexing & Numbering: Top 50 Interview Q&A