Sometimes it is not desirable to replicate all changes that happen in a database to all other nodes in a BDR group. E.g. it might not be convenient and efficient to replicate a table containing session data.
In simple cases, application developers may simply use UNLOGGED tables (a stock PostgreSQL feature). Since such tables don't generate write-ahead-log (WAL) activity, they are completely invisible to logical decoding and thus to BDR. No changes to unlogged tables are replicated.
Where more complex arrangements are needed, replication sets are available.