What is meant by a ‘type’ of certificate?
Certificates can be created for specific purposes, such as signing, encrypting, authentication, etc. For example, S/MIME (Secure Multi-Purpose Internet Mail Extensions) certificates are used for signing and encrypting emails, SSL (Secure Sockets Layer) certificates are used for web-based authentication, VPN (Virtual Private Network) certificates are used for protecting communication from one computer to another on a network.

What is a custom certificate type?
When choosing to create a custom certificate, you can specify a file (template) that determines the exact make-up of the certificate. You can create your own template by using the Certificate Assistant to create a Certificate Authority (CA). The template is available once the CA is created. A Certificate Assistant template is a file with the extension “.certAuthorityConfig” and is located in “~/Library/Application Support/Certificate Authority/”.

What is meant by “Let me override defaults”?
Defaults are predetermined values for a new digital identity that are already assumed by the Certificate Assistant. You can override the defaults by further specifying your own to create your own custom identity.