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[laiocfar]

All of us got a little piece of right.
rookie got promoted to squadie by experience (2 missions or a kill), sergants and upped ranks comes when you got a number of troopers like 5 people = 1 sergant. But the sergant only will be promoted when he got the squadie rank. The upper ranks (anyone over squadie) are given to best trooper avaible with next low rank. So the first captain will be the best of your sergants, nobody skips ranks.
The skills are rised only when you kill aliens but the number and the way that they get up is connected as how you used them. So it usefull to run like fool to get energy, carry a complete arsenal for strengh, to trewh grenades to everywhere for get better at launch, etc.