Any unit whose parts-max
is greater than the default of 1 is a
multi-part unit, and its hp denotes size rather than amount of damage.
Armies and fleets are two kinds of units which can be usefully defined
as multi-part.
Players will very often want to merge or detach parts of a multi-part
unit, and there is an action transfer-part
provided for that.
You can control the cost of the action by setting
acp-to-transfer-part
.