Co was telling us:
Co nous racontait que :

<snip>

You mean transferring the name of "Document A" and some selected text in
"Document A" into a already opened document?

You need code to determine which other opened document to target. What if
there are more than two opened documents, which one will the code select?
What would be the logic?

By using
   ActiveDocument.ActiveWindow.Index
you can get the ActiveDocument window index number.
Then, using some logic you can get the name of the document in another
window.

For example:
'_______________________________________
Dim lngDocIndex As Long
Dim i As Long

lngDocIndex = ActiveDocument.ActiveWindow.Index

For i = 1 To Application.Windows.Count
   If i <> lngDocIndex Then
       MsgBox Application.Windows(i).Document.Name
       Exit For
   End If
Next
'_______________________________________

But if you have more than two documents, you need some logic to determine if
you have got the right one.

Marco,
This code will add text to the first document it finds not saved or if
everything is saved, it will open a new blank document and add the text.

Dim blnFound As Boolean
Dim aDoc As Word.Document
Dim myText As String

blnFound = False
myText = Selection.Range.Text & vbCr & ActiveDocument.Name
'or use .FullName
For Each aDoc In Documents
If aDoc.Path = "" Then
   aDoc.Content.InsertAfter myText
   blnFound = True
   Exit For
End If
If Not blnFound Then
   Documents.Add
   ActiveDocument.Content.InsertAfter myText
End If
Next aDoc