The caption that describes the redaction "for the sake of discretion, brevity, and decency" leads me to believe that they're talking about His Divine Majesty's Most Holy Inquisition. The Inquisition deals primarily with heretical cults, demons, and aliens - things that the average Guardsman is not supposed to concern himself with, save for an appropriate level of hate and fear. Therefore, discussing the Inquisition in any sort of detail is generally taboo.
"Brevity" also points to the Inquisition, as its internal structure is loose and rather complicated. Inquisitors don't have ranks and what organization they have tends to be fairly ad hoc, a matter of assigning resources to the places where they're most needed. Too, the Inquisition is subdivided into Ordos based on their specialty (principally the Ordo Hereticus, which deals with heretics, the Ordo Malleus, which deals with demons, and the Ordo Xenos, which deals with aliens). Obviously the internal structure of the Inquisition is not something a Guardsman needs worry about.
Finally, it's worth noting that the chart is necessarily simplified for the benefit of the poor bloody Guardsman in question. Neither Inquisitors nor Commissars are technically in the chain of command. (Colonel-Commissar Ibram Gaunt being the exception that proves the rule: his dual rank gives him command authority and any number of political and administrative headaches that indicate why that's not usually done.) Of course from the perspective of an ordinary Guardsman, or even an officer, disregarding the "requests" of a Commissar or Inquisitor is likely to end badly, possibly even in summary execution, so as a practical matter the chart is correct.
