Complex sentence: In a book or essay, a more carefully-composed addition is called an afterword, e.g., for a second edition, an afterword, not usually called a postscript, is written in response to critical remarks on the first edition.
Split into simpler sentences: In a book or essay, a more carefully-composed addition (e.g., for a second edition) is called an afterword. An afterword, not usually called a postscript, is written in response to critical remarks on the first edition.