Implicit Intersection Operator: @

The implicit intersection operator was introduced when the Formula Language was upgraded to support Dynamic Formula Arrays.
Before Dynamic Arrays implicit intersection existed and happened behind the scenes automatically.
The @ operator to now used to indicate where implicit intersection could (or did) occur.

link - support.microsoft.com/en-us/office/implicit-intersection-operator-ce3be07b-0101-4450-a24e-c1c999be2b34 

Implicit Intersection

Implicit intersection logic happens when a formula returns multiple values but only displays a single value in one cell.
If the value is a range, then return the value from the cell on the same row or column as the formula.
If the value is an array, then pick the top-left value.
With the introduction of Dynamic Arrays, Excel is no longer limited to returning single values from formulas, so silent implicit intersection is no longer necessary.
If an old formula was invisibly triggering implicit intersection, Excel will prefix the formula with the @ character.

SINGLE Function (Removed in 365)

This function was only provided in Excel 2021 for backwards compatibility and was removed in Excel 365.
SINGLE - Returns the value from a cell range which is the intersection of a row OR a column.
If you try and use this function you will see a warning pop up message and the function will be automatically replaced with the "@" character.
You will no longer get implicit intersection of your formulas, instead, where it detects an implicit intersection, the "@" character or this function will be added.

link - http://benf.org/excel/spill_performance/