The constant is defined in the same file in which it is used. These are defined at the beginning of the file (which also contains a lot of other information). Since the Uniface Service in which the constant is used is quite extensive, with several thousand lines, I didn't find it at first glance.

Another mistake I made was searching with <STATUSERROR>, i.e., with angle brackets instead of simply without angle brackets, such as STATUSERROR in the file.