856 Subfield $u - Uniform Resource Identifier:

Subfield $u contains the Uniform Resource Identifier (URI), which provides standard syntax for locating an object using existing Internet protocols or by resolution of a URN.  Field 856 is structured to allow for the creation of a URL from the concatenation of other separate field 856 subfields.  Subfield $u may be used instead of those separate subfields or in addition to them.

Subfield $u may be repeated only if both a URN and a URL or more than one URN are recorded.