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The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/app-pgbasebackup.html
Description:

The pg_basebackup page says that the --waldir parameter "Specifies the
location for the write-ahead log directory. waldir must be an absolute path.
The write-ahead log directory can only be specified when the backup is in
plain mode."  But it says nothing at all about what it is actually used
for.

You may think it is obvious.  And I thought I could guess - it would be the
directory containing the WAL files for the database, right?   But my attempt
to use it was met with "error: directory "/var/lib/pgsql/Walfiles" exists
but is not empty".  But WAL files are part of the normal operation of the
database, aren't they?  And so the directory _must_ exist, and it is not
surprising it has got something in it!  But that seems to be a problem.  
Which makes me realise I must have been completely wrong and I have no idea
what it for.