Private use character codes and the replacement character have ambiguous width, since they may stand in for characters of any width. 5.0 Conformance East Asian Width is an informative character property. When processing or displaying data Wide characters behave like ideographs in important ways. In fixed pitched fonts, they take up one Em of space. Half-width characters behave like ideographs in some ways.
Narrow characters behave like Western characters in important ways.
The classifications presented here are based on the most widely used mixed-width legacy character sets in use in East Asia as of this writing. In particular, the assignment of the neutral or ambiguous categories depend on the contents of these character sets. Or such a reduction could be done implicitly at runtime in the context of interoperating with Shift-JIS fonts or data sources. Conversely, if additional character sets are created and widely adopted for legacy purposes, more characters would need to be classified as ambiguous. All Private use characters are by default classified as ambiguous, since their definition depends on context.