"UND" is the undiscovered inert element that would be below radon on the periodic chart.
Another type of electron configuration chart is below. These are more commonly known schemes. All you have to do is follow the arrows through the points to find the sequence. Add up the number of electrons as you go, and stop when you have equaled or almost exceeded the number. There have been a large number of variations on this idea, but they all work the same. Arrange the subshells in a slanted order and go through the array in straight lines, as in the first scheme, or arrange the subshells in a straight line and go through the array in slanted lines, as in the second scheme. In these schemes the inert elements appear after the first s subshell and after every p subshell. As the other type, this scheme type has its advantages and disadvantages, but they all lead to the same sequence.