The Extended Text format specification is a simple extension to the traditional text/plain family of file formats. The typical file extension for extended text files is .xtxt and the proposed mime type is text/extended.
The reason for the existence of extended text files is to accomodate the needs of a number of file formats that can benefit from text files that encapsulate multiple streams of text, or put another way, multiplexed text.
The extended text format is based on the utf-8 standard (https://www.ietf.org/rfc/rfc3629.txt)
The extended text format adds a couple of control codes to utf-8, in order to provide the ability of reading and writing files with parallel lines of text.
In multiplexed audio and video formats you have the concept of streams, with parallel content which may be called "frames". In this extended text specification we are not interleaving characters similar to the way some video formats interleave their content. The main purpose of the extended text format is to be able to retrieve parallel lines of utf-8 text. Therefore text characters aren't interleaved in this case, in the case of extended text, we have multiple streams that are recorded as parallel lines of text.
A typical case of an extended text file with multiplexed streams could be a subtitles text file with more than one language.
For example, you could have an extended text file with three multiplexed streams of text like:
# Stream 1 | Stream 2 | Stream 3
# English | #French | Spanish
Hello | Allo | Hola
(Please note that this is not a plain text file. The pipe characters are only for visualisation purposes. A multiplexed file reader would only read parallel text lines from each stream separately)
Another possible example is a rich text format with content separate from formatting and style like:
# Stream 1 | # Stream 2
With freedom and justice for all | [justice] bold
Could render the text as:
With freedom and justice for all
Parallel lines of text are recorded in the extended text file as follows:
[optional utf-8 BOM marker]
Stream 1 text line 1 [NSM] Stream 2 text line 1 [NSM] [NFM]
Stream 1 text line 2 [NSM] Stream 2 text line 2 [NSM] [NFM]
The NSM and NFM markers are two-byte sequences, similar to utf-8's BOM, and identical across all platforms without regard for system endianness.
NSM stands for: Next Stream Marker
NFM stands for: Next Frame Marker