As far as I know, if they say they started processing the application on a certain date, then background checks have not been done yet. They wil not do the checks before they started processing it. The checks are a part of the process. So they will start the checks from Sep 5., not before that. AOR just means, that they have received the application.
BUT:
that does not mean, that you won't get your PR fast, I remember there was someone from the VO Vienna who got his PR within a week after they started processing it. It really depends on each case.