Thanks for your response.
I'm not sure that I understand what you are suggesting.

I added an example code. The function takes the range and returns only
the negative numbers. As it is, the user has to select enough cells and
use it as an array function (Ctrl+Shift+Enter).

Function testit(inputRange As Range)

Dim output() As Double
Dim i As Integer

i = 0

For Each c In inputRange.Cells

If (c.Value < 0) Then
ReDim Preserve output(i)
output(UBound(output)) = c.Value
i = i + 1
End If

Next c

testit = output

End Function