NM, found the answer in the manual:
ORIGINAL: WITPAE-Editor.pdf (Section 3.6.1 page 37)
For many ground units, the AI uses the otherwise unused Port and Airfield fields together to
determine when to change their HQ, as follows:
The Port value is the month the change will happen based on the beginning month in
the scenario (e.g., based on January 42 equal to month 1, January 43 to month 13, and
so forth).
The Airfield value is the Location number of the HQ that the unit will report to after the
change occurs. This allows the scenario designer to set up certain “free” HQ changes
at a specific time in the game for an AI player using these units.