Specifying WAL Location in Streaming Replication

Hi,

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