This field indicates the total number of "user bytes" in the updated control available to be output, or that were already output. By "user bytes", we are referring to characters actually typed by the user, not to any formatting bytes that would be inserted automatically into the output stream based on the output format specified in TXC.DOCFMT'DST.
In the case of plain text output (TXFF_TEXT or TXFF_TEXT_LINES), there are no extraneous formatting bytes needed, so the value can be used to determine the total size. This can be useful when outputting to a string variable using the TXF_DEFERSAVE option, as it would tell you how big of a variable you would need to retrieve the entire contents. But for output formats that include extraneous formatting bytes, such as RTF and HTML, the actual size required will be substantially larger than the value returned in TXC.OUTBYTES. You could, however, use a dynamic (self-expanding) string variable to receive the output, and then determine the actual length using the len() function.
