The service id is used as the keyword match within the local library link resolver. If the library has a subscription to the package, they can set up the link resolver to match on these service ids.
The service id is embedded in the URL that is sent from CWKB to the local link resolver.
For example, this is a real url embedded in the request to the local link resolver. It is a link to the Greek edition of a work on the Perseus platform: svc_id=http://cwkb.org/service/perseus_grk/url:http://www.perseus.tufts.edu/hopper/text.jsp?doc=Perseus:text:1999.01.0203:book=3:chapter=1:section=14
The local link resolver can detect the package by testing the svc_id (i.e., "service/perseus_grk") embedded in the URL. This is most valuable for licensed resources.