Each fief in the game has a unique, five digit code. The first letter indicates the kingdom. Below are all the kingdoms in the game. Note that HRE stands for Holy Roman Empire, a somewhat artificial entity that, in theory, contained the kingdoms indicated.

C Catalonia
E England
F France
A HRE:Arles
H HRE:Germany
I HRE:Italy
R Ireland
M Mallorca
N Navarre
P Papacy
S Scotland

The next two digits indicate the province, as shown below. The last two digits indicate a specific fief within a province (as in 01, 02, 03, etc.)

ADA00 Dauphine
AFC00 Franche-Comte
ANC00 Nice
APR00 Provence

EAR00 Arundel
EBK00 Buckingham
ECG00 Cambridge
ECH00 Cheshire
ECU00 Cumberland
ECW00 Cornwall
EDR00 Derby
EDT00 Dorsetshire
EDU00 Durham
EDV00 Devon
EEX00 Essex
EGL00 Gloucestershire
EHE00 Hereford
EHM00 Hampshire
EKE00 Kent
ELA00 Lancaster
ELC00 Lincolnshire
ENF00 Norfolk
ENH00 Northampton
ENO00 Nottingham
ENU00 Northumberland
ENW00 North Wales
EOX00 Oxfordshire
EPM00 Pembroke
ESM00 Somerset
ESR00 Surrey
EST00 Staffordshire
ESU00 Suffolk
ESW00 South Wales
ESX00 Sussex
ESY00 Salisbury
EWK00 Warwickshire
EYS00 Yorkshire

FAC00 Armagnac
FAE00 Aumale
FAG00 Agenois
FAJ00 Anjou
FAL00 Alencon
FAM00 Angouleme
FAU00 Auvergne
FAX00 Auxerre
FBA00 Bar
FBG00 Bigorre
FBI00 Brie
FBL00 Blois
FBN00 Bearn
FBO00 Bourbon
FBR00 Brittany
FBU00 Berry
FBY00 Burgundy
FCA00 Champagne
FCE00 Clermont
FCO00 Comminges
FCR00 Chartres
FCT00 Carlat
FEV00 Evreux
FFL00 Flanders
FFX00 Foix
FFZ00 Forez
FGA00 Gascony
FGP00 Grand Pre
FGS00 Guise
FGU00 Guyenne
FLD00 Languedoc
FLI00 Limousin
FLM00 LaMarche
FLY00 Lyon
FMA00 Macon
FMF00 Montfort
FMR00 Mortain
FNA00 Narbonne
FND00 Normandy
FNE00 Nevers
FNI00 Nimes
FOR00 Orleans
FPC00 Picardie
FPG00 Perigord
FPN00 Ponthieu et Artois
FPO00 Poitiers
FQU00 Quercy
FRO00 Rouergue
FSA00 Saintonge
FTH00 Thouars
FTL00 Toulouse
FTS00 Tours
FVA00 Valois
FVE00 Vermandois
FVN00 Vendome
FVV00 Vivarais

HBB00 Brabant
HHN00 Hainaunt
HHO00 Holland
HJU00 Juliers
HLG00 Liege
HLR00 Lorraine
HLX00 Luxembourg
HZE00 Zealand

IAS00 Asti
IMN00 Montferrato
IPM00 Piedmont
ISV00 Savoy
ISZ00 Saluzzo

MML00 Mallorca
MRS00 Rosellon