If the number is within the bounds of the list, gives back the element at that position (e.g. 3 gets the 3rd item in the list). But if the number is 0 or less, or greater than the list size, the third parameter will be returned instead.
This is particularly useful for accessing the previous row of a column and avoiding
an error on the first element, for example element or(table\\Table#Totals, row - 1, 0)
in a Calculate will give the value from the Totals column in the previous row, or
0 if this is the first row of the table.