"Pressure Variance in IRC Tank Storage" was engineering regulation 42-15. This regulation for the IRC tank was written by Montgomery Scott in the 23rd century. It was included in the impulse engine specifications.
In 2369, when restoring power to the impulse engines of the USS Jenolan, Scotty admitted to Geordi La Forge that, as a good engineer, he was being conservative when he wrote the regulation and that they should ignore it. (TNG: "Relics")