Targeting is the ability to
LoS extends from the back of a unit's facing for 180° (see Diagram 3). If a model, wall, corner or flamed square is between the unit and the target, then LoS is blocked. Flames are a special case for LoS. A unit can see into the first square of flames (if a unit inside the flames survived) but not anything behind a square of flames.
Firing arc extends from the back of a unit's facing for 90° (see Diagram 4). Firing arc also extends for a certain number of squares in front of the unit based on the weapon being fired, i.e. the weapon's range. For range purposes, diagonals count as 1 square. This 90° arc up to a weapon's range is area that a ranged weapon can be used.
If a unit meets the criteria to shoot at a target, they roll a number of dice specific to their weapon. The highest die is the result. Modifiers are then applied if applicable. If the roll is high enough, the target is destroyed. After this ranged check, the dice are against checked for jams or explosions. So you could have a ranged attack where an Assault Cannon rolls 6, 6, 6, killing its target, then the explosion effect occurs, destroying the shooter.
Melee targeting if different. Close combat attacks can only be initiated against units directly in front of the attacker. Diagonals do not count here. Despite who initiates the melee attack, both combatants roll and compare a number of dice based on their melee ability/weapon. When rolling multiple dice, only the highest number is used, not a sum of dice. Many factors can modify these results, such as weapon bonuses, parries, or blocks. The unit who has the highest end result is the winner. If the winner is facing the opposing combatant, the combatant is destroyed. If the winner (or in a tie) is not facing the other unit, they may turn to face their attacker unit for no AP cost (even making a 180° turn), at the controlling player's discretion. If both combatants tie, then no unit is destroyed and that round's melee is over. An attacker may, of course, initiate another attack with another action point.(Diagram 3) (Diagram 4)
A Space Marine player can react to actions within its LoS by using command points during the Bug Phase. If no action occurs within a unit's LoS, then no action can be taken. An action occuring within LoS does not necessarily mean a unit can fire. Only if the action is also within its firing arc can ranged attacks be made.