In Imperium Nova, each player can recruit one of these three kinds of unit:
Each unit must be based in a particular fortress, specified when the unit is recruited (your initial unit is based in your HQ).
You may move units between fortresses at any time for a small cost, related to the distance between the planets concerned. However bear in mind that an unmanned fortress will have a greatly reduced defensive capacity should it come under attack. Note that if you overfill a fortress with units (not normally allowed but possible under certain circumstances), the units in excess of the capacity listed above will not receive any defensive bonus from the fortress in the event of any attack, as they are caught out in the open and must fight a field battle.
The game will prevent you from ordering any legions to move into a fortress that is already at maximum occupancy. However, it does not limit you from moving in more units than there would normally be room for in a partially-filled fortress. If an indomitable fortress (maximum occupancy: 4 legions) has 4 legions in it, you cannot order any more legions to be stationed there. If that same indomitable has only 3 legions, you can order as many legions to fill that space as you want in the time period between when order the first Legion to move into that fortress, and when that Legion arrives. Only the first Legion to arrive will be stationed in the fortess, and the rest will stay on the planet undefended. They'll be viewable from the Military  screen of the planet, and labeled Overstacked.
A unit can also become overstacked when it returns from an attack or patrol to fortress that is already full, or does not have enough occupancy for the number of returning legions.
Overstacked units stay in “the field” of a planet. This means that during an attack, they would be assaulted first by incoming enemy legions. Since only 5 legions can participate in a battle on each side, up to 5 of the overstacked legions would attack any attacking legions. Attackers do not begin assaulting fortresses until all ovestacked units and patrols have been eliminated.
Clarification - Do additional overstacked units join in the battle when any of the other 5 units die? —Users/DigitalSocrates
All units start with Rookie level experience, unless the House that recruits the units operates in the Military sphere, and builds the legions on a planet that they have constructed a Military Academy on, in which case they start standard. A rookie legions can be trained up to standard with 3-6 maneuvers. A standard legion can be raised up to a Veteran legion with an average of 15-20 maneuvers. Legions can also earn Elite level experience, although this has never been achieved through maneuvers alone. In most cases, leveling a legion to Veteran and above is more cost effective than using maneuvers, although houses that keep standing armies may find using maneuvers to raise standard legions to veteran legions cost effective.