Nisus Writer is a Macintosh word processing program. It was the first word processing software to incorporate multi type systems into one document.
Nisus Writer classic software originally saved to a special file format which separated the plain text into the data fork and the formatting, font information, graphics, sounds, etc into the Resource Fork. Making the file format only compatible with Macintosh HFS systems. The files could be opened with other software, but would only retain the plain text. Files had no extension, but have the Type/Creator of TEXT/NISI.
After Mac OS X, Nisus switched to an RTF based format. They also have a proprietary Compressed Rich Text format, .ZRTF, which has the magic header
1F8B0800 for the first 4 bytes.