Subfield u for uniform resource identifiers has been defined as a valid subfield in many bibliographic fields to provide automated access to electronic resources.
As noted in the OCLC bibformats: "Subfield ǂu defined as the URI is valid in the following fields: 031, 370, 381, 505, 506, 510, 514, 520, 530, 538, 540, 542, 545, 552, 555, 561, 563, 852, 856, 880, 883, 884, and 956. Additionally, subfield ǂu can be used in fields 901-907 and 945-949, either in accord with its control subfield definition or as a locally-defined subfield" (http://www.oclc.org/bibformats/en/controlsubfields.html#subfieldu).
Currently, only URIs in 856 subfield u act as active links. Users should be able to actively access a URI from any valid subfield u in both the record view and the metadata editor.