Regarding:
The INDEX(rng, x, y) function references a range and returns the contents
of the cell located in the x,y position in that range,
where:
x = vertical position in that range
y = horizontal position in that range
Since Sheet1!$A:$A only references a single column,
we only need to identify the vertical postion: INDEX(rng, x)
The CEILING(expression, value) function rounds the expression UP to the
next multiple of value. In this example, the ROW() function returns the
row_number of the cell containing the formula.
The CEILING(ROW()/2,1) function divides the row_number by 2 and rounds
that value UP to the next increment of 1.
Examples:
CEILING(1/2,1) returns: 1
CEILING(2/2,1) returns: 1
CEILING(3/2,1) returns: 2
CEILING(4/2,1) returns: 2
etc
Consequently, every other row increments the referenced range by 1.
I hope that helps.
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