Complex sentence: In this usage, the original connotation was that the attaché was an officer (employee) of another service'attached to,' for example, a diplomatic mission: e.g. the attaché retains a military commission despite being assigned to serve in an Embassy.
Split into simpler sentences: In this usage, the original connotation was that the attaché was an officer (employee) of another service'attached to,' for example, a diplomatic mission. Thus, for example, an attaché who hold a military commission retains that commission despite being assigned to serve in an Embassy.