For multi repository installs we require that own database, own var directory for binary files, solr, own persistence cache and currently also own web proxy is used in order to keep the installs separated.
And while you can somewhat easily do that for file system cache/binary files, database, Solr and even Redis (even if the option for database is deprecated in Redis, you can at least do it). However with Varnish that is not so simple when it comes to tagging.
So we need to take separate tags from one repository to another, and ideally do this transparently.
This can be archived by prefixing tags with configured $repository$ if it is defined (if it is not defined the system picks first configured repository which is usually "default", in this case we can skip prefixing tags).