6.4.3.5 There shall be an implementation-defined subset of the set of char-type values, designated 'characters prohibited from text files'.
This appears to be a loophole provided by ISO/IEC 7185 to allow implementations to get around the requirements specified for text files, especially the requirement that all characters written to a text file appear when the text file is read in.
Irie Pascal designates one character as prohibited from text files. This character is chr(26), also known as the End-Of-File (EOF) character. NOTE: The fact that the EOF character is prohibited from text files doesn't mean that you can't write them to text files, but if you do then the text file will be terminated at that point (i.e. the EOF and the characters written after the EOF will not appear when the text file is read back in). Since this technically violates the standard, EOF's are prohibited from text files.