I think some of the confusion is based on previously used naming standards or conventions for services that use the DNS name space as part of their service name. Previously is was common for services to use prefix under a company's DNS name space. As a poor example, websites www.contoso.com, the www is the prefix and contoso.com is the suffix, this was the same for AD. The suggestion in the name standard documents, that you have referenced, was to give AD a prefix i.e. corp, so the forest root would be dc=corp,dc=contoso,dc=com. Most AD forest don't use the prefix and just use the company's DNS name space. Using this naming convention the term prefix has no meaning anymore, the DN of the forest is the same as the suffix, which is the same as the DNS name space.
With all these documents, they are suggestions, you can name your AD whatever you want, as long as it conforms to DNS and RFC4514 naming standards.
I hope that helps,