The Nintex Forms lookup function is generally used in its standard form with 4 parameters but the runtime function can take up to 6 parameters.  See the link below for the help files for this runtime function.
lookup("list title", "column to filter on", "value to filter on", "output column")

The 5th parameter is a simple true/false boolean to allow multiple items to be returned or not.
By default the 5th parameter is false so the parameter is not required when only one result is required.
When the 5th parameter is set to true multiple results are returned in an Array but what can be done with this Array?

Using the lookup() runtime function on a form with the 5th parameter set to true will return multiple results.
 lookup("list title", "column to filter on", "value to filter on", "output column", true)

These multiple results can be filtered during the lookup by embedding the lookup into other runtime functions.

Examples:
To find the lowest number returned in an array.
 min(lookup("list title", "column to filter on", "value to filter on", "output column", true))
or highest number
 max(lookup("list title", "column to filter on", "value to filter on", "output column", true))
sum all the values returned
 sum(lookup("list title", "column to filter on", "value to filter on", "output column", true))
Average the values
avg(lookup("list title", "column to filter on", "value to filter on", "output column", true))