Specifying WAL Location in Streaming Replication

I am trying to use the streaming replication protocol described in https://www.postgresql.org/docs/10/static/protocol-replication.html to read logical decoding events from a replication slot.

I'm doing this by starting replication with START_REPLICATION, and sending down the most recent position consumed in a standby status update.  When starting replication, I want to begin reading from the last position "committed" by the standby status update.

In the documentation for START_REPLICATION, a required argument is the WAL location to begin streaming at, and I'm not sure what to use here.  I have been using 0, and it seems to work fine.  Additionally, it seems that when --startpos is not provided to pg_recvlogical, it defaults to just sending a wal location of 0.

While this seems to work, I feel a bit uneasy about it since I can't find any documentation explaining why this works or what this argument does when using a replication slot.  Any clarification would be appreciated.

Thanks,

-Matt