The longest code unit subsequence starting at an unconvertible offset that is either: a. the initial subsequence of a well-formed code unit sequence, or b. a subsequence of length one.

• The term maximal subpart of an ill-formed subsequence can be abbreviated to maximal subpart when it is clear in context that the subsequence in question is ill-formed.
• This definition can be trivially applied to the UTF-32 or UTF-16 encoding forms, but is primarily of interest when converting UTF-8 strings.
• For example, in the ill-formed UTF-8 sequence <41 C0 AF 41 F4 80 80 41>, there are two ill-formed subsequences: and , each separated by <41>, which is well-formed. Applying the definition of maximal subparts for these ill-formed subsequences, in the first case is a maximal subpart, because that byte value can never be the first byte of a well-formed UTF-8 sequence. In the second subsequence, is a maximal subpart, because up to that point all three bytes match the specification for UTF-8. It is only when followed by <41> that the sequence of can be determined to be ill-formed, because the specification requires a following byte in the range 80..BF, instead.