The Navy had several types of duty stations and periodicity of assignments.
There were four basic types of duty stations: Sea duty, shore duty, training duty and neutral duty. The first three are pretty obvious. Neutral duty was for assignments that were not as cushy as shore duty, but did not have the separations of sea duty. Assignment to a destroyer or submarine tender was neutral duty, but if the ship was out of her home port for more than 30 days, the time away was reclassified as sea duty.
There were several kinds of periodicity:
Permanent change of station: This was a full move. The Navy paid to move your family and your shit. Actually, if you had a family, you were damned lucky if what the Navy paid covered your costs. A PCS assignment was longer than 26 weeks, but most were between 18 months and five years.
Unaccompanied duty: Your family stayed behind and you went for a year. Before the Iraq War, the most common unaccompanied duty was to the Naval Support Activity in Bahrain and to the USS La Salle (AGF 3), the flagship of the Commander of the Middle East Force, which was homeported in Bahrain. Now, unaccompanied duty includes being sent as an individual augmentee to Iraq or Afghanistan.
Temporary duty: This was usually training duty, where one was sent to a short-duration school. The most common was when you were sent to a school after you left one permanent duty station and were enroute to another. But it could also happen when you were loaned to another command for one reason or another.
Temporary additional duty: These were times when your command sent you away on a trip. It may be to a conference or a short school, ranging from a day or two to a few months.
When you were on TD or TAD, you were paid TDY, which was enough for three decent moderately-priced meals. For obvious reasons, TAD was also known as "traveling around drunk".
This is all background information for some stories, of course.
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