1) Data RAM memory is volatile, it means that if the microcontroller is switched off, or if there is a power failure, then everything in data RAM memory disappear.
Some data values that was known when program was written, may be stored in progrram memory together with program instructions. These may be moved to data RAM when program is started.
Some microcontroller applications will want to store some data permanently,
like alarm clock wakeup time, or temperature thermostat settings, or motor control parameters in a variable frequency drive, to be available also if there have been a power failure, or a maintenance shutdown,
or if a machine is simply switched off at the end of a day.
Such values are typically stored in Data EEPROM.
2) Microcontroller devices are fabricated with different size of memory, for sale at different prices.
When an area in the memory map is grayed out, then there simply is nothing there.