INDEX returns a sub range, specified by row and column number, or an optional range index. Depending on context, INDEX returns a reference or content.
INDEX(Reference; Row; Column; Range)
Reference is a reference, entered either directly or by specifying a range name. If the reference consists of multiple ranges, you must enclose the reference or range name in parentheses. Row (optional) represents the row index of the reference range, for which to return a value. In case of zero (no specific row) all referenced rows are returned.
Column (optional) represents the column index of the reference range, for which to return a value. In case of zero (no specific column) all referenced columns are returned.
Range (optional) represents the index of the subrange if referring to a multiple range.
=INDEX(Prices;4;1) returns the value from row 4 and column 1 of the database range defined in Data - Define as Prices.
=INDEX(SumX;4;1) returns the value from the range SumX in row 4 and column 1 as defined in Sheet - Named Ranges and Expressions - Define.
=INDEX(A1:B6;1) returns a reference to the first row of A1:B6.
=INDEX(A1:B6;0;1) returns a reference to the first column of A1:B6.
=INDEX((multi);4;1) indicates the value contained in row 4 and column 1 of the (multiple) range, which you named under Sheet - Named Ranges and Expressions - Define as multi. The multiple range may consist of several rectangular ranges, each with a row 4 and column 1. If you now want to call the second block of this multiple range enter the number 2 as the range parameter.
=INDEX(A1:B6;1;1) indicates the value in the upper-left of the A1:B6 range.
=INDEX((multi);0;0;2) returns a reference to the second range of the multiple range.