Oblivion Mod:Mod File Format/TES4

TES4 is the header record for the mod file. It contains info like author, description, file type, and masters list.

• Unlike TES3, author and description fields are no longer of fixed size. If you edit their content, you'll almost certainly have to rewrite the entire file.
• In Progress -- some info may be inaccurate.

C	SubRecord	Name	Type/Size	Info
+	HEDR	header	struct	Whether the file is an esm or esp is determined by Flags1 of TES4 record header. If Flags1 & 1, then file is a master. Otherwise, file is a plugin, regardless of what the file extension says.
		version	float	1.0
		numRecords	long	Number of records and groups (not including TES4 record itself).
		nextObjectId	ulong	Next available object id. ?But large variation has been seen?
-	OFST	typeOffsets?	struct[*]	Vestigial—ignored by game executable. Appears in Oblivion.esm. Does not appear in esps. Is not required for esms. You would think that this would be something like offset info.
		offset?	ubyte[3]	Unknown. Looks to be junk data. Not consistent at first, but later in file seem to increment consistently. But is often zero, and if it's supposed to be an offset, it's too small.
		typeNum	byte	Incremental index.
		type	char[4]	Type name. Includes types from the executable that are no longer in the game data, such as TLOD and SNDG.
		unknown	ulong	Always 0, except for type CELL which is very large.
-	DELE	deleted?	??	Unknown. List of deleted formids? Only seen in Oblivion.esm? Note: In Morrowind, DELE records were used to mark data records that had been marked as deleted. This looks different.
-	CNAM	author	zstring	Max size: 512 bytes (including terminator). For files regarded as masters, the CS4 Details view will display "Bethesda Game Studios" regardless of the actual content of this field. The Oblivion plugin loader will display "Bethesda Softworks".
-	SNAM	description	zstring	Max size: 512 bytes (including terminator).
*	MAST	master	zstring	Master filename. Each pair of MAST/DATA subrecords represent a single master of the mod file. Master files are listed in load order at the time the mod was saved. The modindex of formids in the mod file match the order of masters given here.
	DATA	fileSize	uint64	Always 0. Probably Vestigial. In Tes3, the file size of the master was recorded here.