The node identifier can be set to anything you like. I often use this to add a descriptive label to each radio making it easier for me to identify where and what each radio on my network is. The ATND command will list all nodes on your network reporting several parameters including the node identifier.
Another use for the label is the ATDN command. ATDN will search your network for a node identifier and set your destination to that radio if it finds one with the specified name. This lets you simplify your addressing somewhat. +++ ... ATND Servo would switch your destination to a node named "Servo" for example. (note, in API mode ATDN just returns the address of the node. This can be useful for looking up an address.)