[Mobhack]

The game is built on the premise that the currenly selected unit is the "live" one.

The poster may have had difficulty selecting a target for the next selected unit in a mess of wrecks perhaps?. In that case he could use the N and P keys to cycle through (perhaps selecting a more isolated one of the desired formation first, for example), or used either of the HQ menus to choose the exact unit he wanted directly off the force listing.