Can anyone please tell me the difference between VDD,VBAT and Voltage regulator in stm32f407vgt6 discovery board

can you please explain me about the X2 and X3 crystal oscillators in STM32F407VGT6, LQFP100 discovery board and what about SB13,14 not fitted

my question is in STM32F407VGT6 , LPQF100, resistor R26,R27, R28 is N/A and 0..what does it mean exactly i.e.,one resistor R26 N/A is connected to VDD and another resistor R27,R28 is connected to VSS(99pin)..and second question is does VSS has a supp...

where is BYPASS_REG pin of voltage regulator in stm32f407vgt6 and how it was enabled and disabled?

what is the role of voltage regulator in stm32f407vgt6 and how it works..that means already VDD is supplyng 1.8V and again what is the need of suplying 1.2V and how it works i want to know exact operation of it in stm32f407vgt6