The titles listed on the right are the FDLP Basic Collection, it "is a list of titles that every Federal depository library is required to have accessible for immediate use. These titles are vital sources of information that support the public's right to know about the workings and essential activities of the U.S. Government."
In the mid-1990s depository libraries collaborated with the FDLP on a list of titles that were deemed essential to retain in print form (a.k.a. tangable). It was called The Essential List. As government agencies transitioned their publications from print to electronic, ensuring the continuation of the Essential List became difficult.
As of September 2019...
"During the transition to a more electronic FDLP, the (Essential) list reflected GPO’s commitment to distribute certain titles in tangible (in print) format indefinitely. Over time, as agencies transitioned more publications to online-only format, GPO could no longer guarantee tangible printing and distribution of all resources on the list. As a result, the Essential Titles list no longer served its intended purpose.
Libraries looking for key publications that support the public’s right to know about the workings and essential activities of the U.S. Government should refer to the FDLP Basic Collection." Here is the full memo.