We usually refer to Synchronous Replication as simply "sync rep". Sync rep allows us to offer a confirmation to the user that a transaction has been committed and fully replicated on at least one standby server. To do that, we must wait for the transaction changes to be sent to at least one standby, and then have that feedback returned to the master. The additional time taken for the message's round trip will add elapsed time for write transactions, which increases in proportion to the distance between servers. PostgreSQL offers a choice to the user as to what balance they would like between durability and response time.
The user application must be connected to a master to issue transactions that write data. The default level of durability is defined by the synchronous_commit parameter. That parameter is user settable, so it can be set for different applications, sessions, or even individual transactions.
We must decide which standbys should...