From OCLC Bib Formats: "The information required to locate and access an electronic item."
If appropriate, add more than one 856 when the item is accessible in more than one format (pdf, html, etc.) or to give direct access to a part of the item, like table of contents or summary.
1st indicator 4 = http (most cases)
2nd indicator 0, 1, or 2
40 Resource described by bib record
41 Version of resource
42 Related resource (contents, abstract, finding aid, etc.) Note: LC codes Table of contents as 40. When importing, leave as is.
$u Uniform resource identifier
$3 Materials specified (Note: OCLC Bib Formats & Standards differentiates this subfield from $y. Local practice is to use $3 for either the item described in the bib record or a part of the item)
Note: Always make $u the 1st subfield -- it's easier for global changes in Millennium
Links to the resource itself:
856 40 $u [http . . .] $3 Online resource
Links to a version of resource described:
856 41 $u http [. . .] $3 Online resource
Links to a related resource:
856 42 $u http [. . .] $3 Table of contents [etc.]
Links to a serial:
856 40 $u http [. . .] $3 Online resource 1993-Present [etc.]
Links to a particular volume of a multi-volume set:
856 40 $u http [. . .] $3 Online resource v. 3[etc.]
Links to a multi-volume set, not specific volume:
856 40 $u http [. . .] $3 Online resource 2003-2004[etc.]
Links to an item accessed via an online/electronic collection:
856 40 $u http [. . .] $3 Online resource via Early English Books Online [etc., i.e., name of collection]