Challenge 3: A Calculated Column with [@ColumnName] — Solution Walkthrough

OrdersTable has columns Quantity and UnitPrice. A new column,
LineTotal, is added to the table (typing a header name in the empty
column immediately to the right of the table automatically makes it
part of the table).

Formula entered into the FIRST data row of the LineTotal column:
  =OrdersTable[@Quantity]*OrdersTable[@UnitPrice]

What happens automatically:

The moment this formula is entered into just one cell, Excel
recognises it's inside a Table's calculated column and automatically
fills the exact same formula into EVERY other row of the LineTotal
column immediately, with no dragging the fill handle and no copy-
paste required.

Explanation:

The @ symbol inside OrdersTable[@Quantity] means "this row's own
Quantity value, whichever row this formula happens to be sitting
in" — it is fundamentally a per-row reference, unlike a plain cell
address which points at one fixed location regardless of which row
a copied formula ends up in. Because every row's copy of the formula
automatically refers to that SAME row's own Quantity and UnitPrice,
Excel can safely auto-fill it down the whole column the instant it's
typed once — there's no risk of it referring to the wrong row the
way a careless plain-range copy-paste might.

Notes:
- This is the exact mechanism behind this chapter's own tip box:
  [@ColumnName] needs no $ locking at all, because there's no
  relative/absolute row-or-column shifting involved in the first
  place — "this row" is simply redefined fresh for every row the
  formula occupies.
- If a new order row is later added at the bottom of OrdersTable,
  its LineTotal cell gets this same formula automatically too — the
  calculated column behaves as part of the table's own definition,
  not as a formula that happened to be copied down once.