When a unit kills an enemy unit, it can gain experience. A single kill gains the first level of promotion, killing 3 units gains you further promotion. As a unit is promoted, it can gain both bonuses to its stats, and even new abilities.
Elite Shot … (extra shot).
Elite AP … (25% extra action points).
Elite Hunter … (Tanks Sneak faster).
Elite Recon … (Sneak for Free, Automatically turns on Sneak/Hunt mode by default).
Elite Assault … (Assault Cost reduced by 50%).
Elite Overrun … (Tanks can Overrun Infantry in Open)… (Silver and Gold).