A certificate issued to the taxpayer to apply the supplier's sello (signature) to the CFDI (Comprobante Fiscal Digital por Internet) before it is sent to the PCCFDI (Proveedor de Certificación de CFDI) for clearance. It is considered a X.509 soft certificate and you need the E.Firma certificate it to request it. According to Mexican law, PCCFDIs can only clear CFDIs signed with a CSD certificate and should reject CFDIs signed with an e.firma certificate. Go to SAT's CSD web page for more information on how to obtain the CSD certificate.