A certificate the taxpayer uses to authenticate over the Internet to the SAT (Servicio de Administración Tributaria) and other Mexican government services. It is considered a X.509 soft certificate and you need it to request a CSD (Certificado de Sello Digital). According to Mexican law, a PCCFDI (Proveedor de Certificación de CFDI) can only clear a CFDI (Comprobante Fiscal Digital por Internet) 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.