(Answered) How does a unit exit the map?

[Killface]

For what seems on its face so obvious it doesn't even need a specific rule, exiting the map has become the subject of a great deal discussion.

One argument is that all a unit has to do is expend MP to leave the map as if the notional hex it was entering was clear terrain of the same elevation (and water depth) during the movement phase and is out of play, ineligible to fire or be fired upon and irrespective of falls from hip or gyro damage etc at the end of any such move.  For support this interpretation cites TW page 275 regarding forced withdrawal and TO page 215 about rolling maps which both reference units entering half hexes and says that such units are considered to have exited the map and are out of play. 

The opposing argument cites TW page 78, in the section about aerospace units moving from the low altitude map to the space map which states that these units remain on the low altitude map until the end of the turn.  This argument sees this as evidence that when exiting the battlefield via any method, any unit remains until the end phase of the turn in which they announced their departure, therefore vulnerable to fire and any effects resulting from it.

Careful review of the rules turns up no explicit explanation of how to 'exit the map' although the results of it are referenced fairly frequently.

So, how does a unit exit the map?

[Xotl]

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.

[Killface]

The scenario has a group of mechs fleeing from pursuers.  They also have some aerospace cover to try and protect their tail.  When they get to their escape edge a Locust is 7 hexes away from the edge, and walks offmap during movement.  Is he eligible as a target in the combat phase of that turn, or is he out of play the instant he leaves the field? 

the same question applies to the aerospace when they go to leave by flying off their home edge.  Are they eligible targets during the combat phase?

[Xotl]

For scenarios, the specifics of exiting depends on the scenario rules, which could be anything; scenarios are in many cases given free reign to add/alter the standard rules.  I've given the generic rules for such in my previous post, however, and these would apply to both ground and aerospace units (i.e. the moment you leave the map at the point(s) prescribed by the scenario, you're gone, and so would not be eligible targets during the combat phase).

You could easily write a scenario that limits aero units to following the rules on p. 78 in all situations, but that's up to the game runners/scenario writers.