Guys, just check the RR and BGR description and everything would get clear

RR means that u can't get (for ex.) chivalric knights in the begining, you have to wait smth like 60 turns till u got access to them, but BGR allows you to recruit them only when you have one of generals with War Council Member trait in that castle.
TWmadman, RR means RC, because in medieval ages there were only few heavily-armoured warriors on the field, most of them where light infantry. With RC/RR knoghts replenishment time is very high, so most armies aren't fullfilled only with knights (esp. in first 100 turns).