PostgreSQL9.6 Beta1：new setting remote_apply for configuration parameter synchronous_commit
When the standby initially receives the log records, the are written to the standby's pg_xlog directory.  So those records are there for safekeeping, but they have not yet been applied to the shared_buffers and the data files (and the pg_clog, etc.)  Until they are applied, anyone connecting to the standby cannot see the effects of those records.

It is kind of like the difference between getting a note from the post office saying "Your envelope has been delivered" (synchronous_commit=on) and getting a note from your bank saying "Your envelope had been received, opened, processed, and your instructions have been carried out and your account has been credited with the proceeds, have cleared, and are now available for withdrawal" (synchronous_commit=remote_apply).