From Glenn E. Patton:

"To get a sense of which MARC data elements are used in the various indexes for WorldCat, take a look at "Searching WorldCat Indexes" (http://www.oclc.org/support/documentation/worldcat/searching/searchworl
dcatindexes/). Click on "List of Fields and Subfields Indexed in Tag Order" in the navigation bar on the left of the screen to see a list of MARC elements and the indexes in which they are used."