Is there a maximum?????????????????????????????

Would putting a background colour on cells  give a better indication of
which cells you are choosing? It's also easier than typing in a string
of references.

A function  could then go through each cell in a range and just sum
those of particular colours.

Why can you not use
=SUM(C8,B10,D10,D12,B15,G8,F2,E2)

i just want to know how to the SUM function  works. i need a function
that can combine some cells' value in one grid. for instance, there's
A1:Hello
B1:World
C1:=combine(A1,B1) <---- the value of C! will be "HelloWorld"
and the variants in combine() depends on user's selection  then it can
be like this
+--------------+
| A1 |
| B4 |
| G5 |
| ... |
+--------------+

maybe it will like this? but i am not sure.
function combine(a as range)
for each x in ...
combine+=cells(x).value
next x
end function

you dont want a sum, you want to concatonate (sp?) the two values.
Just use a '&' instead of a '+'

=A1 & B1

ya... that's it. but can i use something like =A1 & "\n" & B1?
i want this two elements not in a same line.

if you want to use a line break, you have to use "& CHR(10) &",
which looks like:
Range("c1") = Range("a1") & Chr(10) & Range("b1")

Showing some un VBA experience there

Hmmmm.... I still like my colour idea...

Function fncCombine(rpCombineRange As Range)

Dim rlCel As Range
Dim slCombined As String
Dim slCallerAddress As String
Dim llColour As Long

' What's the colour of the calling cell?
slCallerAddress = Application.Caller.Address
llColour = Range(slCallerAddress).Font.Color

' Go through the range
' Concatonate all cells  with similar colours.
slCombined = ""
For Each rlCel In rpCombineRange
If rlCel.Font.Color = llColour Then
slCombined = slCombined & rlCel.Value
End If
Next rlCel
fncCombine = slCombined

End Function

As far as I experienced, you cannot format cells  (like cahnging
font, colors, etc) within functions. It only works in Subroutines.

I think I noticed from your first posts that you had written something
that looked like jscript or C. A lot of code examples in the microsoft
online docs are given not only in VB(Script) but in JScript and
sometimes C and C++ as well. I thought maybe that would be useful to
you. You could for example choose not to use VBA at all or to have a
look at the examples and compare the code to see how VBA/S does the
same as jscript.

The below link is to some microsoft documentation on the
FileSystemObject. Not relevant to your problem... just something I had
up on the screen when I remembered your posts.

msdn.microsoft.com/.../scrip
t56/html/jsmthfolderexists.asp

try the concatenate function.

=CONCATENATE(C8,B10,D10,D12,B15,G8,F2,E2)

yes, both Colourcell or Concatenate or "&" works fine! :D.
but is there any way to catch which cells  selected by user? i think
the original Sum function  can be a good example, but we don't know
how does it work...still thinking...