VCC: C = Circuit Means Circuit, that is, the voltage of the Access Circuit

VDD: d = device, that is, the operating voltage inside the device; 

VSS: s = series means public connection, commonly referred to as a common ground terminal voltage.

Vee: Negative Voltage Supply; field effect transistor source.

 VBAT: When connected to the VBAT foot using a battery or other power source, the contents of the backup register can be saved and the RTC maintained when the VDD is powered down. If no external battery is used in the application, the VBAT pin should be connected to the VDD Pin.

For digital circuits, VCC is the supply voltage of the circuit, VDD is the operating voltage of the chip (usually VCC & GT; VDD) , VSS is the ground point. For example, for the ARM Microcontroller, its power supply voltage VCC is generally 5V, generally through the voltage regulator module will be converted to the microcontroller working voltage VDD = 3.3 v