ROW(cell) returns the row number of a cell in the spreadsheet. The cell argument is optional and ROW() returns the row number of the cell or row in which the formula is used.
ROW() can be shortened to ROW.
When you enter:
ROW<21 in the select box of a dialog box for a statistical test, only the 20 first cases will be selected.
AND(ROW>20,ROW<41): only cases 21 to 40 will be selected.