Together these contain all of the information relevent to the material in the supercell. The supercell size and shape and the ionic positions within the supercell must be specified. The other cell parameters can be omitted and defaults will be applied in their place. Beware; it is advisable to specify most cell parameters. For example, the default Monkhorst-Pack (MP) k-point grid is unlikely to be suitable for most geometry optimisation jobs.