This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
A Binary II header is a small block of code tacked onto some Apple II files. Sometimes, this is referred to as a "binary wrapper". Mainly, the Binary II header contains filetype information. The purpose of Binary II is to allow Apple II users to download files and have them show up with the correct filetype. (Otherwise, a downloaded file tends to show up as a TXT type file.) In order for a Binary II header to be recognized and used to supply the filetype info, the Apple II downloading software must have its "Binary Down" option set to ON. The problem with this is that, at least on a few popular telecom wares (like Spectrum), setting "Binary Down" to ON will turn OFF Resume Transfer, something you probably do not want to do when downloading over phone lines. When you use a PC or Mac to handle downloads and, then, NULL-modem the files to your Apple, it makes sense to leave "Binary Down" ON for the telecom program running on the Apple II. Resume Transfer is not necessary because you have a direct, noise-free connection between two machines. Meanwhile, Binary Down will automatically recognize and strip-off any Binary II header and save the resulting file with the correct filetype. ----------------------------