Simple means one code point in, one code point out, so conversion is length preserving: U+00DF LATIN SMALL LETTER SHARP S and the ligatures are returned unchanged rather than expanded to SS, FI, ... as SpecialCasing.txt would have it.

As with the classes, the mapping comes from the Unicode Character Database and is therefore the same in every locale --- notably it does not apply the Turkish and Azeri dotted/dotless i rules --- and on every platform, and it works for non-BMP code points also on Windows.