Go back and watch the last few dev streams, it is explained there. How you design your ships determines their role and behavior. Lots of weapons and thrusters make it an attack ship that will attempt to kill high value targets. Lots of defense makes it a defense ship that will attempt to destroy the enemy attack ships before they reach high value targets. Ships with things like support modules or colony modules are considered high value and will attempt to evade attack ships. Large heavily armed ships will be considered capitals and pursue high value targets but target all enemy ships on the way there. The strategy your ships use in battle depends on how you design them and what types of ships make up your fleets.

Well, the way combat works in galciv3 is that later in the game support modules will be a vital part of your fleet, as well as carrier modules. These ships will be very valuable but weaker than other ships, so they will need to avoid combat. And while formations may look cool, it would all go to hell in a matter of seconds anyway =P