Used widely by emulators, SIO2PC & APE peripheal emulators.

DWORD - 32bit unsigned long (little endian)
WORD - 16bit unsigned short (little endian)
BYTE - 8bit unsigned char

16 bytes long.
 

Type	Name	Description
WORD	wMagic	$0296 (sum of 'NICKATARI')
WORD	wPars	size of this disk image, in paragraphs (size/$10)
WORD	wSecSize	sector size. ($80 or $100) bytes/sector
BYTE	btParsHigh	high part of size, in paragraphs (added by REV 3.00)
DWORD	dwCRC	32bit CRC of file (added by APE?)
DWORD	dwUnused	unused
BYTE	btFlags	bit 0 (ReadOnly) (added by APE?)

Then there are continuous sectors. Some ATR files are incorrect - if sector size is > $80 first three sectors should be $80 long. But, few files have these sectors $100 long.