From Destinypedia, the Destiny wiki
On Nessus and Io, the Guards appear as Majors, with those on Nessus guarding Fallen or Cabal prisoners, while those on Io guard Taken prisoner. Once killed, the cell will open, freeing the High-Value Target, which is either killed by patrolling Guardians or manage to escape.
Meanwhile, on Mercury, the guards appear as Ultras, and will always be guarding a Cabal warrior (and a group of Psion Flayers during the Flashpoint. Like their Major counterparts, once killed, the cell will open, releasing the prisoners.
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