This is the battery-compartment that houses the battery.  The standard format holds either the rechargeable battery or the batteryholder for 3 Zinc/Air 675 batteries.  Battery and/or batteryholder have to be purchased separately !!   The Mini format holds the batteryholder for 2 Zinc/Air 675 batteries.  This delivers less power and is therefore not always possible to use with your programs.  You should contact your CI-fitting centre before purchasing the Mini version !!