Final answer:
When unsure about a symbol on a P&ID, it should be looked up on the master sheet, which provides definitions for all the symbols used in the diagram.
Step-by-step explanation:
If you do not know the meaning of a symbol on a P&ID (Piping and Instrumentation Diagram), you should look it up on the master sheet. The master sheet, also known as the legend or symbol sheet, is part of the P&ID documentation suite and contains the definitions and representations of all the symbols used in the P&ID. It is essential for anyone working with these diagrams to understand the symbols to ensure proper interpretation and implementation of the design.