LIRIS data can be saved in three different ways using the storemode command.

The bias and dark level of near-IR detectors is usually not stable over time so a bias image (the so-called pre-read) is taken immediately before the actual exposure (the post-read). There is nothing one can do with the pre-read apart from subtracting it from the post-read, so the recommended setting for the store mode is diff:

SYS> storemode liris diff

Saves a FITS file with one extension. It contains the difference between the post-read and the pre-read, i.e. the "bias-subtracted" image only is stored. This is the recommended storemode.

SYS> storemode liris diff_pre

Saves a FITS file with two extensions. The first contains the difference between the post-read and the pre-read (i.e. what you would get using storemode diff), the second one the pre-read itself (i.e. a separate "bias").

SYS> storemode liris pre_post

Saves a FITS file with two extensions. The first contains the pre-read and the second the post-read.