A1 (TRUE, 1, or omitted): The address format should use letters for columns and numbers for rows.
R1C1 (FALSE): The address should use the R1C1 format, where R1 is the row number of a cell and C1 is the column number.
The given address can be a range reference, such as "A1:C5", not just a reference to a single cell. If used this way, INDIRECT returns an INDEX