When the literal or named constant specified on a bound call starts with "CEE" or an underscore ('_'), the compiler will treat this as a system built-in. (A bound call results with either CALLB or with a prototyped call where EXTPGM is not specfied on the prototype).
If it is not actually a system built-in, then a warning will appear in the listing; you can ignore this warning.
For more information on APIs, see the iSeries Information Center programming category. To avoid confusion with system provided APIs, you should not name your procedures starting with "CEE".