Hi,

I just want to make sure I understand the terminology here.

With my little understanding about "a register", it is indeed memory address; has 2 kinds of memory data register and memory address register. However, it is not something user level code can modify like these I2C unique address, command address, etc. However, I kept reading they were referenced as registers. Thus, I am bit confused here. Could someone clarify that for me?

Register and memory address are often used fairly interchangeably. I would use register for I2C, since I would consider memory address something in the RAM of the chip. The register is what you access through the I2C command and might not really be a memory address at all, but rather a trigger to do perform an action, like "start calibration".

Regards,
Xander

Xander is correct, and in fact, PMBus, a specification for power supply
management over I2C, specifically avoids the term "register" in favor
of "command" to avoid just this kind of confusion.

When you perform an I2C transaction you are really just sending a
number over to a slave device, and then writing or reading some data.
That number may be an offset in a device's memory, or it may be
something to be interpreted by a slave device that's running software itself.