Are asking about the Top() function?. Top() is an chart inter-row function, like above() and below(). The inter-row functions compute an expression using the dimension values on "another" row in the chart. The difference between the inter-row functions above(), below(), top() and bottom() is primarily the starting orientation -- which row of the chart.
Top() could be used in a somewhat similar way to rank(), but not exactly. The top() row is defined by the sort order of the chart. Rank() has no connection to the order of the chart. RownNo() is sort of the chart row equivalent to Rank().
-Rob
