Grid Certificates are for both servers and clients who participate in the global e-Science Grid.
Client Certificates at the moment can be 'manually' obtained via CertCentral.We are working Work on the integration of SAML federations in the Digicert DigiCert portal is in progress.
Document Signing is for signing documentson behalf of an organization. Client certificates can also be used but there are differences, certainly in a PDF environment using Adobe Acrobat.