So it's clear that if you use the main domain, it's directly example.com, while for the subdomain, the domain name becomes blog.example.com, and one more change that also occurs in the way the DNS is set, the subdomain only needs to change the Cname side in your domain's DNS management. For more information, check at s.id/subscription

Now subdomains can be used in custom domains. This service is only available on the ULTIMATE package. So you can use a subdomain from your original domain to create a custom domain.