MI6 stand for **Military Intelligence, Section 6**. As the name implies, there *are* other sections. [According to Wikipedia](https://en.m.wikipedia.org/wiki/Directorate_of_Military_Intelligence_(United_Kingdom)), there were originally 17 of them (1-19, with MI13 and MI18 not being used), though many were absorbed into various other organizations.
Their duties vary from administration (MI1), mapping (MI4), propaganda (MI7), and interrogation of POW’s (MI19).
EDIT: MI5 deals with domestic intelligence, similar to the FBI, while MI6 is for foreign intelligence, similar to the CIA.
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