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ITC states for the Behind Enemy Lines secondary

Behind Enemy Lines​: 1 point if at the end of your player turn you have 2 or more of your units at least
partially within 12” of your opponent’s rear most and longest board edge

What edge is this for the Search and Destroy Deployment? Does it extend across the entire edge, even if that edge ends up outside my opponent deployment zone?

You need to ask ITC. House Rules are House Rules.

Yes. The longer deployment edge, just like with vanguard strike, even if it is outside of their deployment zone.