(extension (optional "www.w3.org/PICS/service-extensions/label-bureau" "URL-for-label-bureau"))
A client which sees something of this form in a service description should interpret it as meaning that URL-for-label-bureau is the URL of a label bureau to contact to request labels for this service. Because the term optional has been supplied, a client which sees a line of this form but which is unfamiliar with the extension is free to ignore this line. Several lines of this form may be included in a single service description, indicating several label bureaus that have this service's labels.
The following products have announced the ability to interpret this "label-bureau" extension:
The following products are known not to interpret this "label-bureau" extension: