The yellow exclamation sign notifies the user that the respective remote Host version is outdated relative to the Viewer version. For example if Viewer is version 6.10.10 and a remote Host is 6.9 , the sign will be shown for the respective Host icon.
Note that the sign only shows if the Host version is outdated as compared with the Viewer's. So it isn't shown if you have the same outdated version on both ends. For instance, the current stable version is 6.10.10. If you have version 6.9 installed on both Viewer and Host end the sign won't be shown. In order to know for sure that you have an outdated version you should visit our website or just enable checking for updates in Viewer options.