campaigns.
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Units survive the troops get tougher. they can buy more gear and gain skills.
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They can also get injured.
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If taken out of action. roll d6. on 6 dead. on 5 injured. skills can help with this.
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If survive the unit gets 
<li>1xp per troop that survived.
<li>1xp per wound caused. incl glancing or penetrating vehicle hits.
<li>any bonus for objectives met, role playing or MVP.
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This can then be spent on the unit. 
To buy better weapons, to gain skills, to add new troops.
<br>Skills can only be used by a unit if all members have it. So if new members join to fill losses they will have to have the units skills bought.
<br>The xp can be passed up the chain of command?
<br>
xp should be used where it makes sense. eg don't gain new assault skill after a pure shooting game. 
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Also note that new equipment and skills may change the Force Organisation position of a trooper or unit (eg if get more than 2 skills) which may increase its cost to use in games (eg if need 1 more elite slot).
<p>
Often in campaigns reinforcements will give a number of points able to be spent on troops and or equipment.