=A2
=B2&" , "&A3
Sub test()
Dim c As Range, x As Long, t As String
x = Range("A" & Rows.Count).End(xlUp).Row
For Each c In Range("A1:A" & x)
t = t & c & ","
Next
Range("A" & x + 1) = Left(t, Len(t) - 1)
End Sub
In addition, it assumes that there is a sequence number for each one in Column A. If that's not the case, please let me know.
As you can see, the sequence number is not listed again until it is changed.
Sub Concatenation()
Dim x As Long, tmp As String, c As Range, cv, nv
x = Range("B" & Rows.Count).End(xlUp).Row
For Each c In Range("B1:B" & x)
cv = c.Offset(, -1)
nv = c.Offset(1, -1)
If IsNumeric(cv) And cv <> "" Then
tmp = "K" & Right(c, 3)
Else
tmp = tmp & Right(c, 3)
End If
If (IsNumeric(nv) And nv <> "") Or c.Row = x Then
c.Offset(, 1) = tmp
tmp = ""
End If
Next
End Sub