usually when a person has been on a
job a while he knows what it consists of. He then should write up his
hat, meaning in this
case a
folder which contains
past orders and directions which outline his
job plus his own summary of his
job. When one is transferred or leaves a
post he is supposed to "write up his
hat" which is to say, modernize this summary of the
post.
(HCO PL 3 Dec 68)