The Conceptual Structure page purports that:

In HCS, A Security Label is defined as a data structure composed of four Named Tag Sets that record security classification, security category, security control (also known as handling caveat), and security trust.

I was not able to find the fourth category in HCS. Moreover, in the detailed specifications page, only three groups are covered: security classification, security category, security control.

It seems to me the fourth group "security trust" is a remnant of some content that was later removed from the IG in the course of reconciliation.