Confused about a statement in WAL configs

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/wal-configuration.html
Description:

In this section:

"At checkpoint time, all dirty data pages are flushed to disk and a special
checkpoint record is written to the log file. (The change records were
previously flushed to the WAL files.)"

I am confused by what the part in parenthesis means by "previously". This
may only reflect my ignorance, but I don't know if "previously" means "in
previous versions of Postgres" or "before the checkpoint" or something
else.

Mostly because of this, I don't understand what this statement means. Thank
you.