I think this should get you started. This replaces the RXXXX with the Aaa,BBB(1000) style and also gets rid of the BU_zzz. You'll have to explain a bit more where else you want to go from there. Hope this helps.

Sub custom_delete()

    Dim totalrange As Range
    Dim check As Range
    Dim LastRow As Long
'Find the last used row in a Column: column A in this example
    With ActiveSheet
        LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
    End With

Set totalrange = Range("A1:A" & LastRow)

For Each check In totalrange
    
    If InStr(check.Value, "R") = 1 Then 'check if it starts with R
        'take the value of the first cell above it which normall has the
        'Aaa.BBB(1000)-style code
        tempstring = check.End(xlUp).Value
        
        check.Value = tempstring
        check.End(xlUp).Clear

    ElseIf InStr(check.Value, "BU_") Then 'check if the value of the line starts with "BU-"
        
        check.Clear
    
    End If

Next check


    
End Sub