It means you are reading something that is refering to a
Sensitive unit but because of the classification of the document
the Unit can only be referred in Numbers.
e.g. A unit is Secret and its operations is best not known to the
regular joe. But that unit have to write something to let the
regular joe know that they exist. So in a "Restricted" Level
document, they are referred to as HQ 1234.
This usually happens when that unit have to install something
within the compounds of another camp, but that unit have to let the
RP and Guard personnel know that there will be frequent traffic in
and out of that camp from that unit.
So that unit writes to the RP, "HQ 1234 have installed a Gundam
Detecting radar in . HQ 1234 will frequently book into the camp to
check the condition of this radar. HQ 1234 have notified RSM of and
personnel will enter your camp at 1000hrs every wednesday in 12345
MID. Please allow the below personnel from HQ 1234 to
proceed"