What does a double line typically signify regarding the instrument's location?

A double line commonly signifies that the instrument is located in an auxiliary location. This representation is used in instrumentation and control diagrams to differentiate between primary controls or instruments and those that serve supplemental or secondary functions.

In many schematics, primary instruments might be designated with single lines or other symbols, while auxiliary instruments, which may support the main process without being directly part of the primary control system, are denoted with double lines. This helps operators and engineers quickly identify which instruments are core to the operation and which ones are supplementary, aiding in effective troubleshooting and operational strategies.

The other options offer scenarios that do not align with this common industry practice; for instance, stating that an instrument is stored in the office or is part of the main control system would not typically be represented by double lines. Similarly, while a backup system is important, it is generally indicated through different symbols or designations entirely, separate from the standard auxiliary notation.