inventory setup questions

I have some inventory setup questions when it comes to average costing. Specifically these fields.

Automatic Cost Posting.

Automatic Cost Adjustment

and Average Cost Period.

Now, the Automatic Cost Adjustment field only matters if the Automatic Cost Posting is turned on correct ?

Also, let’s say I post some inventory increases.
+10 in jan.
+10 in feb
+10 in march.

now its July and I post a Sales Invoice for -15. In NAV, any decreses get applied against any increases. but how far back does NAV go ? Is that what the Automatic Cost Adjustment field is for ? To determine how far back to apply my decrease to my increase ?