What does Approve To Amount field represents for

Hi All,

In the Invoice Approval rule, in Detail Tolerance section, there is a field called "Approve to Amount". My initial understanding is this is what the amount user can approve at max limit on the invoice.

In my case,
My Shipment cost is 500 USD

In the approval rule profile, i mentioned,
Default Tolerance Section,
Selected Apply Cost Approval Rule check box.
Allowable Percent Above is 10.
Allowable Percent Below is 5.

And in the Detail Tolerance section,
Approve To Amount = 500 USD
Allowable Percent Above = 10
Allowable Percent Below = 5

I have attached this to the Service Provider.

My Shipment cost is 500 USD

Then in case,
if my invoice cost is 480 - it is approving
if my invoice cost is 510 - it is approving
if my invoice cost is 520 - it is approving
if my invoice cost is 530 - it is approving
if my invoice cost is 540 - it is approving
if my invoice cost is 550 - it is approving
if my invoice cost is 560 - it is not approving.

Then what is use of Approve To Amount field.

Thank you.