There are lots of theories. One is that they are the first letters of cities conquered by Romans. A second theory is that apparently in the Old Imperial German Court the walls of the Royal Mews/Stable yard were initially marked with letters indicating where each Courtier and/or riders horse was to be stood to await its rider.
K – Kaiser/King
F – Furst/Prince
P – Pferdknecht/Ostler
V – Vassal
E – Edeling/Ehrengast/Guest of Honor
B – Bannertrager/Standard Bearer
S – Schzkanzier/Chancellor of Exchequer
R – Ritter/Knight
M – Meier/Steward
H – Hofsmarshall/Lord Chancellor
No one really knows though!