Question about the registers of an ADC
1 - The ADC is given a spot in the same memory the CPU uses to park its result. It isn't a separate chunk of memory. Same memory, reserved location.
2 - In an 8 bit CPU, memory comes in groups of 8 bits. I presume your ADC is 10 or 12 bit? Thus, it is greater than 8 bits but less than 16. The micro gives you the choice it should put the ADC results towards the left or right of those 16 bits. For example, for a 12 bit ADC: left justified will use bits 15-4. Right justified will use bits 11-0.