The HP 10bii calculator determines IRR/YR for a set of cash flows using mathematical formulas that search for the answer. The process finds a solution by estimating an answer and then using that estimate to do another calculation. This is called an iterative process.
In most cases, the calculator finds the desired answer, since there is usually only one solution to the calculation. However, calculating IRR/YR for certain sets of cash flows is more complex. There may be more than one (or no) mathematical solution to the problem. In these cases, the calculator displays a message to help interpret what has happened.