The xll library can report errors, warnings, and information using pop-up alerts. These can be turned on or off using any sum of XLL_ALERT_ERROR(), XLL_ALERT_WARNING(), or XLL_ALERT_INFO() flags. The function returns the previous mask and the argument is stored at HKEY_CURRENT_USER\Software\KALX\xll\xll_alert_level in the registry to persist across Excel sessions.