It refers to the voltage that a battery sitting unused longer than 3 days should be charged or discharged to.
The "Storage Voltage" or "Nominal Voltage" is the most stable voltage for a LiPo battery to sit unused. Think of it as the "sitting" or "idle" voltage.
11.1v is the Home Original storage voltage
14.8v is the Home Plus battery storage voltage
See the math:
Storage voltage for LiPo batteries is 3.7v per cell (multiplied by the number of cells in the battery pack)
Home Original battery has three internal cells 3x3.7v=11.1v
Home Plus battery has four internal cells 4x3.7v=14.8v