ERF file extension - SGML Visual DTD entity reference file
What is erf file? How to open erf files?
File type specification:
SGML was originally designed to enable the sharing of machine-readable documents in large projects in government, law and industry, which have to remain readable for several decades—a very long time in information technology. It has also been used extensively in the printing and publishing industries, but its complexity has prevented its widespread application for small-scale general-purpose use.
Primarily intended for text and database publishing, one of its first major applications was the second edition of the Oxford English Dictionary (OED), which was and is wholly marked up using an SGML-like markup.