... The NICMOS data reduction and calibration pipeline employs file formats and software architecture techniques that are new and different from what has been used for previous HST instruments. This paper describes the FITS file format used for NICMOS data, which includes error estimate and data quality arrays for each science image, and describes the approach used to associate multiple observations of a single target. ... This group of five data arrays is called an image set, or ``imset'' for short. ...