PESEL, or Powszechny Elektroniczny System Ewidencji Ludności, is Poland's unique 11-digit identification number for registered individuals. It comprises the birth date (YYMMDD), a unique serial number (PPPP), and a checksum digit (K) for validity (e.g., YYMMDDPPPPK). This number is crucial for employees in Poland to manage their tax obligations (PIT-37) and can be obtained without a registered address. Applicants must complete a form (in Polish) and submit it to the city hall in their district with identity proofs and a legal reason for needing a PESEL. Although not mandatory for foreigners, having a PESEL facilitates tax and social security documentation for employers.