fadden
commented
2 days ago I don't know if the format was documented outside Apple. archive.org is currently in a bad way, so no help there for the moment.
Update: John Morris confirmed the lack of published documentation, and said he had to reverse-engineer it for the Applesauce tool. He noted:
From what I remember, the structure was fairly straightforward except for the sector data being grouped into chunks of up to 20960 bytes. Figuring out the "best" compression level drove me nuts for quite a while. The "better" compression is just an RLE.
(Very low priority)
CiderPress II v1.0.4 is unable to parse or decompress the files found inside Apple II Software - Apple TIS 3.0 1992.zip. They are disk images in the DART format. (This zip file is an excerpt from the Apple Technical Information Source CD (Tech I. S. 3.0) - September 1992 which also contains the vintage DART 1.5.1 software that presumably generated them.)
DiskCopy 6.1.3 is able to decompress them. This can be easily verified at Infinite Mac - that link will open a vintage Mac emulator in the browser with the above-mentioned CD automatically mounted. Disk Copy can be found in the emulator's mounted volume "Infinite HD" under the folder "Utilities". Launch that app, and use its menu item "Image > Convert Image" to select one of the DART images in "Tech I.S. 3.0 / Apple Software Library / Apple II Software". Saving the converted file to the "Outside World / Uploads" folder will cause the web browser to download the result as an uncompressed .po file, which of course CiderPress is able to view with no problem.