What does the ISA 5.1 first letter X stand for?

In ISA 5.1, the first letter X stands for Unclassified. It can also stand for Custom, one-off measurements not covered by other letters.

What second letters commonly follow X in ISA 5.1 tags?

Common combinations include XT (Transmitter, per project legend), XY (Computing element).

X - Unclassified

Q: What are example ISA 5.1 tags starting with X?

XY-901 (Computing element for an unclassified service per the project legend).

Reference for the ISA 5.1 first letter X. Treat X as the escape hatch you use when you've exhausted every other option. If a measurement is plausibly classifiable under another letter, classify it that way. X tags propagate downstream as unfiltered noise that the I/O list owner has to chase. Legacy drawings tend to accumulate X tags as drafters ran out of conventional letters. Modern drafting practice keeps them rare.

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Also stands for.

Custom, one-off measurements not covered by other letters

Common second letters.

Transmitter, per project legend

Computing element

Example tags.

XY-901

Computing element for an unclassified service per the project legend

Typical placement.

Wherever the project legend defines.

Related

GlossaryISA 5.1 ISAISA letter N - User's choice ISAISA letter O - User's choice