From the introduction:
ACE is an international agency that operates in environments both openly hostile and clandestine. ACE is a third party spy network with elite operatives and a web of connections spanning the entire world. Basically if a problem is large enough to force the inclusion of ACE, it is on an apocalyptic scale.
ACE Agents are international players in the field of espionage. Allied with no country, their work benefits the entire world rather than a single nation. Acting as allies with other intelligence agencies, ACE agents focus on areas that require the help of a third party rather than problems that can be solved internally. However the case for new and young recruits is often different, with them being tossed into less world-shattering problems so that they may become more venerable.
Perhaps due to their status as non-government and non-affiliated making ACE Agents seem expendable to outside agencies, or perhaps because of the reputation of ACE Agents capabilities, agencies that work with ACE have a tendency to put any ACE Agents into the worst situations. Generally this equals a very low survivability rate. This has cultivated a “golden rule” among ACE Agents.