Limitation on the “Computer Name” in the “Advanced Options” on the AD-NT auth server configuration is due to the NetBIOS protocol’s limitation on the Hostname.
PCS runs on a Unix type operating system and needs to contact the Active Directory server (Windows Based Operating System) in a Windows domain based networked environment for the purpose of authenticating users. For working in tandem with an Active Directory Server, PCS uses Samba protocols which runs over the NetBIOS protocol.
NetBIOS protocol has a limit in length of hostnames, as per the NetBIOS Naming convention. NetBIOS names are 15 characters or less in length. This is the reason there is a limitation in the PCS on the maximum length on the “Computer Name“ field.
Refer to the link for further details on NetBIOS Names.http://www.tech-faq.com/netbios-names.shtml