The NL(Filter) function returns a string value that can be used as a filter in another NL function. It's intended use is for filtering the contents of one table based on the contents of another table.
Example of the NL(Filter) function
The following formula will retrieve a list of records from the Orders table:
NL(Filter) can be used to filter on a field that is not on the Orders table (such as the "Subtotal" field on the “Order Subtotals” table):
The inner NL(Filter) function returns an array of the values of the OrderID field where the value of the Subtotal field is greater than 100. The Jet Excel add-in can use this array as a filter for the Orders table.