This depending on where you are a CA admin, and how the certificate will be used.
Option A: In large corporate networks, usually have a Domain Controller os SSO (Single Sign-On) setup using Microsoft Active Directory and so on. In this case, you can set up a CA server that in a more automatic way will issue certs that are trusted on your "domain", basically your intranet ecosystem of Active Directory and the PCs that connect to it, directly or through VPN connections.
Option B: If you are a CA admin of a hosting company where SSL/TLS Certificates need to be trusted over the internet by the public, you need an independent certificate authority's certificate that is trusted by browsers by default. For this, you can only use the CSR ("Create Certificate Request...") option, since you need to provide the CSR text to the Certificate Authority. They will then return the signed certificate that you install on IIS. Using the "Complete Certificate Request...".