The so-called "Mark" of Calth relates to two things. The first, refers, as per XIIIth Legion combat record protocol, to the elapsed time count (in Terran hours [sidereal]) of the combat. All Ultramarines operations and actions of this period may be archivally accessed for study, and their elapsed time count mark used as a navigation guide. An instructor might refer a novitiate to "Orax mark: 12.16.10", meaning the tenth second of the sixteenth minute of the twelfth hour of the Orax Compliance record. Usually, this count begins at either the issuing of the operation order, or the actual operational start, but at Calth it was timed from the moment Primarch Roboute Guilliman ordered the Ultramarines to return fire against the traitorous Word Bearers Legion. Everything before that, Guilliman said, wasn't a battle: it was merely treachery. Even in the latter 41st Millennium, while the Word Bearers still live, in the madness of the Maelstrom or in the depths of the Eye of Terror, until they are all hunted down and destroyed, the Mark of Calth will continue to run.
Secondly, the "Mark" of Calth refers to the solar radiation burns suffered by many of the combatants, principally the human (specifically non-transhuman) troops, when the Word Bearers caused Calth's sun to go supernova, bathing the planet in deadly radiation. The survivors on the surface were forced to retreat into Calth's massive cave systems where they took refuge within the massive arcology-cities. Here the battle would continue, against the remnants of the Word Bearers who had also taken refuge within the cave systems, in a decade long campaign known as the Underworld War. The last of these veterans to die, many years later, still refused graft repair and wore the mark proudly.
- Know No Fear (Novel) by Dan Abnett, pp. 19-20, 312-313