Correct way in Expression Builder

[dindigul]

I tried to do a simple calculation (on a form) but fail to understand this:
=[UnitPrice]*[Quantity] works, however if I use the Tables list from
(below) and construct the formula it shows as:
[OrderDetails].UnitPrice * [OrderDetails].Quantity
and it does not work. I get #Name error
When field names are written as [UnitPrice] is it not a standard practice to
name them as such? Somewhere I also see [OrderDetails]![UnitPrice] notation.
Why? Thanks