From the BEAT website:

BEAT is excited to announce a new initiative to link LC bibliographic records to full text electronic copies of these same cataloged materials residing in collections of other institutions. Though these works, all in the public domain, have been digitized by various institutions, many of the original printed works are also in the collections of the Library of Congress. By linking LC catalog records to these electronic versions the Library expects to provide users with more unified and centralized access to materials of this nature as well as provide users of the LC collections or of LC catalog data rich and substantive information about the contents of these works as well as access to their texts.

In order for records to qualify for enhancement in these projects, the electronic versions have to be an exact version of a print version represented in the Library’s collection, as established by the presence of an Library of Congress Control Number (LCCN) for the electronic version that matches the LCCN for the print version.

In this project, the LC catalog record is modified to add an 007 field indicating a computer resource, to add a note for the existence of a digital reproduction (in a 530 field presenting information that the item is available in a different physical form), and to create an 856 linking field that points to a meta-level description record at the holding institution. In turn, that meta record provides links that allow the researcher to browse the full text itself. These operations are accomplished as a result of a program written by David Williamson, cataloging automation specialist. Some but not all of these modified LC catalog records are redistributed, depending on whether access points and other details are in conformance with AACR2.

Thanks to Amy R. for the info.