Tag prefixes are not always used. When they are used it is usually to differentiate a set of tags in one area from a similar or identical set of tags in another area. P&ID drawings are usually arranged so that tags from two different areas are not shown on the same drawing sheet, so tag prefixes shown on one sheet are usually all the same. For this reason it is recommended that a note on the drawing sheet be used to indicate that all tags shown should be prefixed with the given number, rather than repeatedly show the same number prefixing every tag on the sheet. A lot of drawing "real estate" can be saved by using such a note. In this case, a prefix can still be added to each tag during tag export. See Exporting Tags .

P&ID Tags Reader  will recognize a tag prefix on an instrument balloon in a drawing if it meets the following conditions:

  1. The prefix is a number.
  2. The prefix is found near to, but outside the top left quadrant of the balloon