Image to be used as android system partition. In Halium, this is un-sparsed abd uploaded to /data partition using adb:
simg2img halium-klte-2019-07-09-system.img halium-klte-2019-07-09-system.img.raw
adb push halium-klte-2019-07-09-system.img.raw /data/system.img
Or by using halium-install script.