Quoting the manual:

Bibliography mode (bibl-mode) is a subsystem of GNU Emacs which allows you to keep track of information on the World Wide Web and elsewhere. It is primarily intended for storing information about software packages, Web pages, mailing lists, and other data of interest to users of the Internet, but can also be used to keep references to books and papers, and can be extended to hold a heterogeneous database of most any sort of textual information.

BiblMode is used by RememberMode to keep track of URLs and BookMarks.

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