This message indicates that the Interval Global Solver has systematically explored the solution space and has found a solution that is very probably the global optimum, but it has not been able to “prove global optimality.”  Most often, this means that there is more than a tiny difference between this solution’s objective value and the best bound on the global optimum’s objective value that the Solver has been able to find.  For more information, see the discussion of “Interval Global Solver Stopping Conditions” in the section “Limitations on Global Optimization.”