Rule 1. Once a Clan unit attacks enemy unit, no other Clan unit may may engage that target. Only if original Clan attacker is destroyed, or under one of the conditions laid out in Rules 2 or 3 can another unit attack that target.
Rule 2. A Clan unit may engage any unit that has fired on it, even if this contradicts Rule 1.
Rule 3. Attacks against Capital ships are exempt From Above Rules. Any number of fighter units or DropShips can engage single WarShip.
Rule 4. A clan unit will, by preference and ideology always engage a single enemy unit until that unit is destroyed or Rule 2 occurs. Attacking more than one target simultaneously changes the nature of the engagement from a mass of one-on-one duels to a huge "grand melee" (no restrictions). Few Clan commanders are willing to open a battle up to a grand melee. The punishment meted out to warriors who initiate such melees (if they survive) is severe.
Rule 5.If Clan AeroStar engages a smaller unit, it will use offensive firepower appropriate to the number of fighters in the smaller unit
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Forces prosecuting the Trial of Annihilation will use maximum force available...

Rules of Engagement, or RoE, is the preferred method of combat for trueborn Clan Warriors. Trueborn are the product of the eugenics system of the Clans. They are created artificially in laboratories, and genetically engineered to produce the ultimate Warrior. In contrast, freeborn are people who have been given birth the natural way. According to the way of the Clans, only the trueborn are superior and pure. They, by birth right, are with honor, and righteous. Among the elaborate and mysterious ways of the trueborn Warrior is the Rules of Engagement.

RoE was a method of combat devised after the Exodus from Terra, to minimize huge losses of valuable equipment and human lives in combats and battles. The most important aspect of RoE is single opponent combats. The brief general rules of RoE are as follows, and pertains only to BattleMechs vs. BattleMech battles:

A Warrior will never fire on a 'Mech already engaged with another opponent. This means that no double teaming or gang banging allowed.

A Warrior will at no time initiate physical attack, such as charging, punching, kicking, or clubbing.

Only in the event that a Warrior has no more weapons at his or her disposal, may he or she engage in physical attacks.

Following these basic guidelines ensure that a Clan Warrior will fight with honor. There are other conditions to RoE, that individual Clans will follow. For example, depending on the Clan, a Warrior may or may not be able to:

Fire on a retreating 'Mech

Fire on a shutdown or fallen 'Mech

Fire at the rear of a 'Mech

Target the head of a 'Mech
Failure to adhere to RoE during combat will cause loss of honor for a Warrior, as well as serious repercussions from his or her commanding officer. The use of IDF, or indirect fire, will also result in a loss of honor.

However, in the event that the enemy fails to follow RoE, one may return the dishonor. For example, if the enemy attacks using physicals, the Warrior may return the physical only to that enemy. In the event where the enemy as a whole act dishonorably, the commanding Clan officer may call a Grand Melee, at which point the battle field becomes a free for all--a Warrior may at this point use any means at his or her disposal to terminate the enemy.

To a true Clan Warrior, RoE is the path for honorable combat. However, strictly following RoE does not ensure honor, and there are other implicit aspects as well. From my personal examples and first hand experience with RoE, there is more to Clan combat than just terminating your enemy. A true Clan Warrior knows when to grant hegira--the chance for an enemy who has proved his or herself to be a worthy opponent to withdraw from the field of battle without further combat or losses. In addition, true Clan Warriors will also try to challenge themselves by battling the strongest and toughest of the enemy. Success under greater odds are a testament to a Warrior's skills and honor.

The only other exception to these Rules of Engagement are when engaged with mercenaries. Mercenaries do not combat for a worthy cause. They seek only material and monetary advancement, and so they are, by definition, without honor, and deserve no RoE. Elementals are not 'Mechs, so they are not required to follow RoE.