Who Can Strike?
A model can fight if it's in base contact with an enemy model when its turn to strike comes, even if the two bases only touch at a corner. Models attacked in the side or rear can still fight back -- they're assumed to twist around as best they can in the crush of battle, without physically turning to face their attacker.
Normally a model can only strike at an enemy it's in base contact with (the main exception being a supporting attack, below) -- but any model may always direct its attacks at rank-and-file models, as long as it's in base contact with any model of that unit, including characters and command group models. A model with more than one Attack that's in contact with two or more enemies of different profiles must choose which one to attack before any dice are rolled.