Well, AFAIK, this variable is set when there are warnings or non-fatal errors during installation. And these revolve around missing files and / or folders, a file / folder already being there or not present, a file that cannot be copied / moved / deleted etc.
IA_INSTALLER_STATUS value is set by InstallAnywhere. For each action, install anywhere gives a Status depicting the result of action. If the action has an ERROR or Warning due to various reasons then the IA_INSTALLER_STATUS variable is set accordingly.
In order to determine the cause of error see the installanywhere log after you are done with the installation. This should tell you the cause of error.
