here's a brief explanation:
SetDef is composed of off-world insurgents who seek to control the solar system. They see themselves as the true inheritors of the solar system, and believe their way of life to be "stronger, more pure," than the people of Earth.
In SetDef's mind, living off-world has made them more worthy of ruling the solar system, while those who reside on Earth are weak "fat cats," living off the bounty of the off-world colonies.
They value functionality over style, an attitude reflected in everything from their ships to their weapons and armor, which is modular, with everything designed to protect them from the harshness of space. They have re-purposed service robots into military roles, adding to their legions of troops. For SetDef, a true victory would mean the utter cessation of all planetary resources reaching Earth. They are willing to kill civilians, soldiers, and anyone who gets in their way.