In Apache conf's Servername setting, I have to specify a directory.

For example Servername world wide web.example.com/directory. I trid and it doesn't work, though.

I've got a primary site (world wide web.example.com) which documentroot is /var/world wide web/html/primary and my sub site URL is world wide web.example.com/sub and it is documentroot is /var/world wide web/html/sub.

Thanks.

world wide web.example.com/directory isn't a valid value for ServerName. Only hostname and port could be incorporated within the value for ServerName.

What you need to define is known as an Alias. You are able to find out about Alias here.

For instance, inside your configuration for world wide web.example.com, you can include this line:

Alias /sub /var/world wide web/html/sub