This is most useful in emulating some board games,
or to prevent clever players from exploiting a mess of move costs.
The default of -1
is the most playable,
since player will always be able to use all of their mp.
Otherwise, there may be situations in which a unit has
a few acp left, but not enough to go anywhere,
and so they end up being wasted.
The free move does not actually get subtracted from the unit's acp,
it just doesn't let lack of acp forbid the move.