For L<public-inbox-v2-format(5)> inboxes, this value is
multiplied by the number of Xapian shards. Thus a typical v2
inbox with 3 shards will flush every 3 megabytes by default
-when unless parallelism is disabled via C<--sequential-shard>
+unless parallelism is disabled via C<--sequential-shard>
or C<--jobs=0>.
This influences memory usage of Xapian, but it is not exact.