Polish car plates - the system
Poland is divided into 16 voivodships, each has a letter ascribed. Voivodships are subdivided into powiats, or counties, with their own letters. Now, combine major with minor letters, add some numbers and random letters and you get a Polish registration plate.
PO means a car from Poznań.
POB means Oborniki county close to Poznań.
POS, Ostrów Wielkopolski county.