I am just studying Excel 2007 VBA programmer's Reference book. On that chaper 1: Primer in Excel VBA there is a UDF example as below.
In the command line InvoiceAmount = Price * DiscountVolume + Price * _
(1 - DiscountPct) * (Volume - DiscountVolume)

what the command (1 - DiscountPct) refers. What the command meaning to (1 - DiscountPct). Can someone explain to me?

Function InvoiceAmount(Product, Volume)

'Create object variable reffering to table in worksheet

Set Table = ThisWorkbook.Worksheets("Sheet1").Range("A2:D5")

'Find price in table

Price = WorksheetFunction.VLookup(Product, Table, 2)

'Find discount volume threshold

DiscountVolume = WorksheetFunction.VLookup(Product, Table, 3)

'Appy discount if volume above threshold

If Volume > DiscountVolume Then

'Calculate invoice with discount

DiscountPct = WorksheetFunction.VLookup(Product, Table, 4)
InvoiceAmount = Price * DiscountVolume + Price * _
(1 - DiscountPct) * (Volume - DiscountVolume)
Else

'Calculate invoice without discount

InvoiceAmount = Price * Volume
End If
End Function