Battery for System Time
All XC3 and XD3 cores have a built-in battery slot inside. When the core is delivered from the factory, this slot is empty.
A NTP server provides a proper system time for the core, if no battery is installed, the system time will be lost after a power cycle.
When you insert a CR2032 standard button cell battery into that slot, the system time will be kept even after a power cycle.