The concatenation operator (&) is only applicable to strings.
If any value in the expression is a string, the result of the operation is a string concatenation as illustrated in the following code example:
$script.subtotal = 14
$script.subtotalText = "Subtotal: " & $script.subtotal
The value stored in the Script object variable subtotalText would be the string value:
"Subtotal: 14"
Using an empty value ("") in an object variable or as an explicit value in the expression causes string concatenation.