Taken from the file plugins.h in the Zoom distribution.

/*
 * The Zoom archive format is:
 *
 * (char) Magic1
 * (char) Magic2 - or - (char) Magic2B
 * (char) Magic3
 * (char) Magic4
 * 
 * IF Magic2B was received THEN
 *		(long) logicalEof /* For multi-file archives * /
 * END IF
 * 
 * <EntryChain>
 * 
 * The format of <EntryChain> is a linked list of
 * EntryInfo, where "next" points to the next logical address
 * on disk. "next" as 0 means no more entries.
 *
 * For a directory, the "creator" field points to the
 * first file/folder entry inside the directory.
 *
 * For a file, IF the "what" field is ZOOM_PLUGIN,
 * the EntryInfo is followed by a length byte and that
 * many characters naming the compression engine.
 * Right after that (or right after the EntryInfo in the
 * case of uncompressed files or default compressed files)
 * follows the data fork compressed, followed by the
 * resource fork compressed.
 *
 * Note that there is no "end of compressed data" marker;
 * your compressor engine will have to figure that out by
 * itself. You could for instance do an ftell before
 * compressing; writing a (long)0 and then write your
 * compressed data, seek back and write the actual length.
 *
 * Note that new entries always are added last in the file,
 * so you need not worry about overrunning anything else.
 */

/*
 * The default compressor in Zoom is the same as used in
 * "better" compression mode in ZOO 2.10. A Zoo extractor
 * or convertor could parse the ZOO header format, and use
 * the built-in engine for "lzh" compressed files.
 *
 * The simplest way to do this is to call SetEngine(-1) and
 * call Encode / Decode. -1 is the default compressor, 0 is
 * the null compressor (fork copy)
 *
 * Likewise, a UNIX zoom packer/unpacker could use the source
 * for zoo 2.10 functions "lzh_encode" and "lzh_decode"
 * (they're wrappers) for compression.
 */

/*
 * This "EntryInfo" is presently also a file header.
 * Some fields may be non-obvious. Don't use these.
 * For instance, "comment" is currently unsupported,
 * and should be left as 0
 */
#ifndef ZOOM_TYPES
typedef enum zoomWhatType {
	ZOOM_NOTHING , ZOOM_FILE , ZOOM_UCFILE , ZOOM_DIR , ZOOM_PLUGIN
} ZoomWhatType ;
#define ZOOM_TYPES
#endif

/*
 * Remember to fill in "hlen" correctly as well. When reading a header,
 * Zoom checks with this field to see if it should skip some more data
 * or seek back a little, so as to make future field additions at the
 * end possible. You should NOT add your own fields to this structure.
 */
typedef struct EntryInfo {

	/* "what" is a ZoomWhatType */
	char			what ;			/* Negative if deleted */
	unsigned char	hlen ;			/* Header length */
	unsigned short	boolFlags ;		/* Boolean flags */
	long			next ;			/* Next entry */

	long			complen ;		/* Length of compressed data */
	long			compdata ;		/* Data fork portion of compressed data - for dirs, number of entries */
	long			uclen ;			/* Length of uncompressed entry */
	long			ucdata ;		/* Data fork part of uncompressed */

	long			type ;			/* File type */
	long			creator ;		/* File creator - for dir, offset of file */
	long			mdate ;			/* Modification date */
	long			comment ;		/* Comment offset */
	short			flags ;			/* Macintosh file flags */

	short			dataCrc ;		/* Data fork crc */
	short			resCrc ;		/* Resource fork crc */

	unsigned char	name [ 32 ] ;	/* File/Dir name */

} EntryInfo ;