According to B on Discord in Prowlarr support channel:
"The flow is like this: no tags on application = sync all indexers, no matter what other tags the indexers have. app has at least one tag = only if common tag found."
"ApplicationLongTermStatusCheckSingleClientMessage":"Applications unavailable due to failures for more than 6 hours: {0}",
"ApplicationStatusCheckAllClientMessage":"All applications are unavailable due to failures",
"ApplicationStatusCheckSingleClientMessage":"Applications unavailable due to failures: {0}",
"ApplicationTagsHelpText":"Sync Indexers to this application that have no tags or that have 1 or more matching tags",
"ApplicationTagsHelpText":"Sync Indexers to this application that have one or more matching tags",
"ApplicationTagsHelpTextWarning":"Tags should be used with caution, they can have unintended effects. An app with a tag will only sync with indexers having the same tag.",
"ApplicationURL":"Application URL",
"ApplicationUrlHelpText":"This application's external URL including http(s)://, port and URL base",