Download Orders - Download new orders from your online store into eCC. This should be configured based on the frequency with which you expect to receive new orders in your store.
Post to QuickBooks - This settings tells eCC to post the order to QuickBooks at your specified time intervals. In case you want to edit the pre-conditions for posting to Quickbooks (recommended), click settings, which is next to post to QuickBooks.
Retrieve tracking ID - This feature automatically retrieves the tracking id from QuickBooks, at specified intervals. This works only if you have the QuickBooks Shipping Manager enabled and in use.
Post to Store (notify customers) - This should be configured based on the frequency of your orders and how often you want to post back updates to your Store.