One of the rights of the noble classes is the right to field private armsmen. Of course, because these nobles are subject to the King, those armsmen may be called up in times of war, so long as they are trained and equipped to certain standards. In return for maintaining their personal guards and having them at the crown's disposal, the crown gives the overlords of these units "House Guard Commissions", which may be activated in times of war.
For example, the Earl of Quelquechose has extensive holdings, and maintains a force of two hundred and fifty private guards. He appoints his sons head of the various portions of his guards. If these guards are maintained to standard and war breaks out, the King can order the Earl to place his men at the disposal of the crown. In the meantime, the unit exists on paper as say, 2nd, 3rd and 4th companies of 2nd Battalion, 9th of Foot.
In the event of war, the Earl is allowed to appoint any baneblooded adult male he wants to the post of Lieutenant-Colonel of 2nd Battalion, 9th of Foot (because he would be supplying most of the men in 2nd Battalion). He would also be allowed to appoint the commanders of 2nd, 3rd and 4th companies, which would likely end up going to his sons. During this time of war, the men are paid, fed and equipped out of the State coffers. The Earl and his sons are subject to military regulations as well. In addition ,they are not allowed to sell their commissions. Should a spot become vacant through other means (death or disgrace), the post may be sold, with the money going to the previous holder of the commission or his families.