The Last X register is an invisible one that will store the last value that was used from the X register in a mathematical operation. The value is recalled by pressing ālā (lowercase ell) or clicking the LastX button.
Example:
3 Enter 4 +
This will yield the answer of 7 in the X register, and the Last X register will contain the last value from the X register, namely, 4.
This can be used as a primitive undo function, provided you know which operation you did last, and that there is an inverse operation. For example, immediately following the example above, you can enter:
l - l (ell, minus, ell)
This will restore the stack to the state it had just prior to the addition. (For a complete undo, use Alt-Backspace or Ctrl-Z).