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There are several national variants of ASCII. As a result, the original ASCII is often called US-ASCII. ISO/IEC 646-1991 defines a character set, similar to US-ASCII, but with code positions corresponding to US-ASCII characters @[]{|} as national use positions [ISO/IEC 646-1991]. It also gives some liberties with the characters #$^`~. In particular characters (e.g., #$^`~). In ISO/IEC 646-1991, several national variants of ASCII are defined, assigning different letters and symbols to the national use positions. Consequently, the characters that appear in those positions, including those in US-ASCII, are less portable in international data transfer. Because of the national variants, some characters are less portable than others: they might be transferred or interpreted incorrectly.
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