This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
In a DOS 3.3 Catalog sector, the third byte in each file's entry tells the type of the file: Byte Value* File Type 00 Text 01 Integer BASIC 02 AppleSoft BASIC 04 Binary 08 S type 10 R: Relocatable object module 20 new A type 40 new B type *DOS 3.3 sets bit 7 of the byte if the file is locked. (e.g. 84 --> a locked Binary file) Type R files show up in just a few applications. An R file begins with 6 bytes which a "loader" routine can use to tell the Target location of file contents, How many bytes to move, and Source location to move from. Although S, new A, and new B are included, no official application was defined for them and no DOS commands were created to make any special use of these files. --------------------- from David Empson There was an "R" type relocating loader included with the toolkit for use with BASIC programs and relocatable routines being loaded into upper memory. "S" was used by some programs for a generic image file, or something that was not likely to be touchable with normal code. The LISA assembler used the second "B" type for its source files. It had a patched version of DOS that changed the file type list to read "LARSBAIT", so the source files appeared in the catalog as "L" if you booted LISA, or "B" if you booted a normal disk. The "B", "A", "R" and "S" special file types cannot be accessed by BASIC programs (unless you patch DOS) - commands are only provided for dealing with "B", "A", "I" and "T" files. The four special types can only be accessed using direct calls to the File Manager. ___________________________ By: Dave Althoff, Jr.