青丘 is the name of the home planet of the Foxians (and Borisin)
As for slave caste in the military... how it worked is that in Borisin society, when you're born they check your biology. If you're considered genetically inferior (such as having foxian biological traits, I guess, or have any sort of defect), then you go into the slave caste.
But there are also individuals in the slave caste who show to have martial ability. Those individual are generally taken into the military and conditioned into spies and shock troops to use against the Xianzhou. They're the reason why military SOP for the Yaoqing when going up against foxian shocktroopers is to take no prisoners, even if they claim to surrender. It's also the reason why the most infamously cruel and malicious of the current Borisin warpacks is actually made up of Foxians (brainwashed slaves who want to prove themselves to be more Borisin than their masters).
no subject
As for slave caste in the military... how it worked is that in Borisin society, when you're born they check your biology. If you're considered genetically inferior (such as having foxian biological traits, I guess, or have any sort of defect), then you go into the slave caste.
But there are also individuals in the slave caste who show to have martial ability. Those individual are generally taken into the military and conditioned into spies and shock troops to use against the Xianzhou. They're the reason why military SOP for the Yaoqing when going up against foxian shocktroopers is to take no prisoners, even if they claim to surrender. It's also the reason why the most infamously cruel and malicious of the current Borisin warpacks is actually made up of Foxians (brainwashed slaves who want to prove themselves to be more Borisin than their masters).