The MAC ID is a hardware identification that is unique for each device (for our purposes) this MAC ID never changes...think of it as a serial number for the ethernet portion of the device.
The IP address is intended to be changed and there are a number of schemes to acoomplish this.
For most PLC networks, you will want to end up with a fixed IP address that doesn't change dynamically once it is set (unlike the office PC world where it's no big deal to wait a few seconds to get a dynamic address from a server).
So a brand new device from A/B will normally come "out of the box" with BootP enabled. BootP is a way for the device to send a specific type of message out on to the wire and say "Hey, I need an IP address, is there a BootP server out there that can give me one?".
The BootP server, if one exists on the network, looks for these types of messages (which will include the MAC ID) and lists them, allowing the user to pick the item from the list that matches the MAC ID of the device that needs assignment. The user then chooses the IP address desired, and the BootP server sends a message to the device saying "Okay, device with MAC ID x, here is your IP address", and then should indicate that the device accepted the IP address successfully. It also contains a mechanism to turn off the BootP requests so that following the next power cycle, the process does not start over, but the assigned IP address is stored and used indefinitely.
The Rockwell BootP server has been reported to be less than 100% reliable in some particular situations. I would have to search for the threads where this has been mentioned, but for me, it has always worked as advertised.
This is an oversimplified street talk description but hopefully helps with a basic understanding. Hopefully, one of the network gurus can come along and expand on the details and correct any mistakes I may have made in my simplified explanation.