When a control-program function has been successfully completed, a return value of 0 is returned to the calling thread. A non-zero return value indicates that an error has occurred.

Each non-zero value corresponds to a symbolic error identifier that indicates the cause of the error. For example, a return value of 2 from DosOpen (indicating that the file was not found) corresponds to the symbolic identifier ERROR_FILE_NOT_FOUND.

DosErrClass helps applications deal with non-zero return values by taking a return value as input and returning both an error classification and a recommended action. Depending on the application, the recommended action could be followed, or a more specific recovery routine could be executed.