Parameter -1 - what does it mean?

[Ricky B]

You are talking movement costs - at least for a hexside -1 is impassable. I am guessing 0 may be used for hex movement cost when impassable, versus an impassable hexside. But 0 wouldn't work for hexside movement costs, as that would be a valid cost for some hexsides, versus a hex movement cost where valid would always be greater than 0.