The question about EKF2

Earth Magnetic Field Vector - (North, East, Down) can converted from Body Magnetic Field Vector - (X,Y,Z)
by [magN;magE;magD]=Tnb*[magX;magY;magZ].

Anybody know why Earth Magnetic Field Vector - (North, East, Down)
and Body Magnetic Field Vector - (X,Y,Z) are the state vector of EKF2 at the same time?

Anyone know about it? Thank you for your reply!