This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
xB was a term used in the late 2390s to refer to former members of the Borg Collective operating on the Artifact. Like the renaming of the cube itself, it was meant to be a first step towards the establishment of a new identity that left the Borg past behind. (PIC: "The Impossible Box")
According to Hugh, who identified as an xB himself, the xBs were the most despised people in the galaxy. xBs were seen either as property to be exploited or hazards to be warehoused. However, the Romulan Free State had a more expansive vision for the xBs found on the Artifact, seeing them as both. (PIC: "Maps and Legends", "The End is the Beginning")