if two armies end up in a space, regardless of reason, then one is eliminated - with appropriate SW penalties, as per 5.24. however, the rule is completely mute on which army is eliminated.
for example, AoP is in manassas, AoC is in fredericksburg. AoNV is in hanover. the csa attacks the AoC, which is defeated, forcing AoC to retreat. if manassas is the only legal retreat, which army is eliminated? AoC or AoP? who decides, the csa or usa in this example? the significance is that different SW penalties may apply, depending on which army is eliminated...
Owning players choice...