SIE is, I believe, an American Air Force-unique acronym.
SIE = self-initiated elimination.
SIE may mean the individual’s performance is purposely not up to expectation, in other words, they intended to fail, compelling someone to relieve them of their obligation.
SIE also references an individual’s deliberate request to be removed from training or a mission.
The former, purposeful performance not up to expectation, is viewed negatively. The latter is more often viewed positively given an element of intellectual consciousness and self-awareness is involved.
Too many people SIE negatively; they just do not want the work or responsibility that accompanies truth, intellectual consciousness, and self-awareness.