Scorpio is a SRU Term that gives an officer (usually the sniper) the permission to fire his weapon/use lethal force. This command is given by the Team Sergeant. Why is this particular word used for the KILL order, though? One reason may be that, like the NATO phonetic alphabet system, where a word starting with a particular letter indicates that initial letter itself (example - Sierra is used to designate the Sniper because it starts with 'S'), Scorpio begins with the same letter as the word ' Shoot'. However, my guess is that the star sign Scorpio is used because that sign is attributed to the card in the Tarot deck whose title is 'Death', and when a sniper shoots, he or she shoots to kill. Also, the scorpion is a very poisonous animal whose venom is powerful and often causes death - thus, it is good symbol of lethality.