A transport is a ship that carries soldiers to an enemy planet and then initiates a planetary invasion. Any ship outfitted with a transport module is capable of fulfilling this role. Upon launching from a planet, a dialog box will ask the player the number of soldiers he/she would like the transport to carry; these soldiers will be removed from the planet's population. A transport can carry 500 soldiers for every Troops Module and 1000 soldiers for every Adv Troops Module installed.
Right-clicking the blue ship notification icon that appears upon completion of a transport will launch that ship from orbit with a full troop compliment.
Soldiers work the same way as colonists. Any time a transport ship enters orbit around a planet, the soldiers disembark and join the planet population. As soldiers are recruited from the civilian population, you can pick up troops at any planet and not just the one that built the transport ship. When launching large scale invasions while playing with specialized planets, it can be useful to have your construction worlds producing the transport ships, then launching them with only one troop on board and picking up soldiers from nearby high population planets. This will allow the populations to recover much faster than if you recruited all of your soldiers from one world.
Core ship design