the same as the rest of them, they will supply a feed for amplifiers, the front is usually used in the front speakers, my stereo has 6 RCA sockets, 3 red 3 white, all up, --- a seperate set one for subs, one set for fronts and one set for rears, just make sure the 'fade' is set to zero, like front to rear, or it might not sound right,
Yes, the front RCAs can be used for the rear speakers, it just means that if you have front speakers running from your head unit, you won't be able to fade front or back. You'll have to set the gains on the speaker amp to match the front speaker volume.
Can I ask why you're amplifying your rear speakers but not your front speakers? Stereos always sound better with good front speakers amplified and then the rears off the head unit or amplified. Your front speakers should produce the most sound... Having rears louder than your fronts is like going to a concert and facing away from the band