See also: Multiplayer rankings
Unit experience table
If you recycle an experienced unit, in addition to gaining 50% of its price back, the next unit you manufacture will gain its experience. This doesn't have to be one at a time: You can recycle an army, then later manufacture a new army with the old army's experience.
Your "saved" experience will be kept across campaign missions and saved games—if you do not manufacture new units before you win the current mission, you can manufacture new units on the next mission that have the upgraded experience.
Commanders increase the experience bonuses of units they command.
If a unit has a commander, it gets bonuses. If the commander has a higher level than the unit, the unit gets bonuses as if it had the experience level of its commander. Otherwise, the unit gets bonuses as if it were one level higher than its current level.
For instance, if a Green unit has a Professional commander, it gets bonuses as if it were a Professional unit. If a Special unit has a Professional commander, however, it would get bonuses as a Hero unit.
The special unnamed Hero + 1 level cannot be reached by gaining experience—only by assigning a Hero unit to a commander.
The information regarding commanders here applies only to 2.3 and later.