A certificate the taxpayer uses to authenticate over the Internet to the Servicio de Administración Tributaria (SAT) and other Mexican government services. It is considered a X.509 soft certificate and you need it to request a Certificado de Sello Digital (CSD). According to Mexican law, a Proveedor de Certificación de CFDI (PCCFDI) can only clear a Comprobante Fiscal Digital por Internet (CFDI) signed with a CSD certificate and should reject CFDIs signed with an e.firma certificate. Go to SAT's e.firma web page for more information on how to obtain the e.firma certificate.