ActiveSyncVirtualDirectory and Interna/External URL

I've been reading this blog, in preparation for our Exchange 2007 to 2013 migration. In step 6, the author provides the command:

Set-ActiveSyncVirtualDirectory –Identity "e2k7ServerName1\Microsoft-Server-ActiveSync (Default Web Site)" –ExternalUrl $Null –InternalUrl https://legacy.contoso.com/Microsoft-Server-ActiveSync

I don't understand why I'm setting up the InternalURL property for ActiveSync. Looking at our existing virtual directory (multi-tenant) we have Internal =https://serverFQDN/Microsoft-Server-ActiveSync and External =https://mail.domain.com/Microsoft-Server-ActiveSync. Which way is correct?

Thanks.