LBB5'80, p43:
BOARDING
_ _ Disabled enemy ships may be captured by boarding. In order for boarding to take place, two conditions must be satisfied.
First, the ship to be boarded must be disabled; it must be incapable of maneuvering, all of its offensive weapons must be disabled, and it must not have a working black globe generator.
_ _ Second, it must be separated from protecting friendly ships; this is assumed to occur if, at any point after the ship is disabled, the owning player has the initiative and changes range from short to long (retreating, in effect).