Ground units can't willingly leave the map (including moving into incomplete hexes) unless the scenario or special rule allows it. In the case of a scenario, unless it says otherwise then moving off the map would occur in the Movement Phase and cost a base 1 MP (yes, for all we know the unit falls and kills itself at the end of that move with a PSR as you mention, but a scenario doesn't care about such things). Involuntarily moving into an incomplete hex or otherwise off the playing area results in that unit being considered destroyed for that game (unless you're involuntarily moved off the edge of the map that the scenario tells you that you needed to move off anyways, in which case you're good).
Aero units, being separate unit types with their own ruleset, use their own rules. Do not take page 78 as a blanket ruleset for any unit type in BT.
With those caveats given, can you explain the specific situation under consideration? As there's no single universal rule, I'd need to know which unit type and ruleset/subrule you're talking about when you're asking about this.