The Mark is an ability possessed by the Hush of Paradeisos, and is technologically implanted in a Sinner upon their ascension to that role. It is a derivative mimicry of the Shackles, allowing the Hush to exert psychological control over marked individuals. In addition to other Sinners, it was seemingly possible for Shalom to Mark the Chief as well (though she declined to do so). Similarly to the Shackles, Marking an individual seems to require direct skin-to-skin contact between the user and the target. Shalom most often Marks a Sinner while in the process of using her personal Sinner ability on them, but it is not clear whether this is a requirement to activate the Mark ability. Marking a Sinner also appears to require the target to share a similar emotional profile with the user. In Shalom's case, the prerequisite feelings are anger and defiance.
The Mark is less overtly powerful than the Shackles, but more insidious. A Marked individual may not even be aware that they were subjected to its power -- Christina wonders in N.F. 113 whether she had been under Shalom's influence since the first time they met in N.F. 104. Meanwhile, the more a Marked Sinner uses their own ability, the more their consciousness becomes subjugated to the will of the Marker. Donald spent nine years without using his ability for this reason, but still found it nearly impossible for him to attack Shalom.
The Mark technology remains in active development. Shalom is likely the first Hush to possess the ability, as it seems to be based on the Shackle technology that was acquired by Paradeisos during the Eclipse Operation, which was the previous Hush's final mission. Her Schorl stated that the using the Mark entailed "high stakes and resource consumption," and thus Paradeisos preferred for most Sinners to be restrained with the Shackles instead. However, Schorl also believed that by the eighth or ninth generation Hush, the Mark would have improved enough for the Hush to take over the duty of detaining Sinners.
Marking "Dissenters" seems to be one of the primary purposes of the Hush, but it is unclear whether this word holds special meaning, or is just a term to describe Sinners who are opposed to the will of Paradeisos.
Paradeisos was forced to temporarily suspend usage of the Mark technology at the end of the Blockade Operation due to Shalom's "silent rebellion". With the Observatorium destroyed and no solid information on how the system was compromised, Paradeisos feared that the Manic power which the Mark imitates may have been responsible for contaminating the Hush.