Compare with XML's entry line, the "XML Declaration", encoded itself in UTF-8 or UTF-16 (only used once, which should really be enough; does one really want to switch to another encoding in the middle of a file, which demands appropriate editor support?). See: Autodetection of Character Encodings (Non-Normative)

<?xml version="1.0" encoding="UTF-8"?>

In this case for example:

:- encoding(utf8).

"in the remainder of the file" is too vague for this use, but likely means "after the dot + EOL of this directive".