Skyrim 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.

• Flags indicate whether a file is a master (Flags & 1) or plugin, regardless of what the file extension says.
• Flags also indicate whether a file has localized string tables (Flags & 128). If this flag is not set, lstrings are treated as zstrings.

C	SubRecord	Name	Type/Size	Info
+	HEDR	header	struct
		version	float	0.94 in most files; 1.7 in recent versions of Update.esm.
		numRecords	int32	Number of records and groups (not including TES4 record itself).
		nextObjectId	ulong	Next available object ID.
-	CNAM	author	zstring	Max size: 512 bytes (including terminator).
-	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.
-	ONAM	overrides	formid[]	Overridden forms This record only appears in ESM flagged files which override their masters' cell children. An ONAM subrecord will list, exclusively, FormIDs of overridden cell children (ACHR, LAND, NAVM, PGRE, PHZD, REFR). Observed in Update.esm as of Patch 1.5.24. Number of records is based solely on field size.
+	INTV	unknown	uint32	Unknown. Possibly "internal version"? It's unclear if this subrecord is required. Observed in both Skyrim.esm and Update.esm.
-	INCC	unknown	uint32	Unknown. Introduced with the Skyrim 1.6 update in Update.esm.