Finally, I figured it out with the help of Gjaa code above; I've got the final code as below; with this code you can control the contents of the richTextBox1 from Button1 or any tab of the tabControl:
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
RichTextBox1.Clear()
For s = 1 To 100
RichTextBox1.AppendText(Label1.Text & " - No. - " & s & vbCrLf)
Next
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
RichTextBox1.Clear()
For s = 1 To 100
RichTextBox1.AppendText(" Button2 - " & s & vbCrLf)
Next
End Sub
Private Sub TabControl1_SelectedIndexChanged(ByVal sender As Object, _
ByVal e As System.EventArgs) Handles TabControl1.SelectedIndexChanged
Dim indexOfSelectedTab As Integer = TabControl1.SelectedIndex
Dim selectedTab As System.Windows.Forms.TabPage = TabControl1.SelectedTab
With TabControl1.SelectedTab.Controls
.Add(RichTextBox1)
.Add(Label1)
.Add(Button1)
End With
If indexOfSelectedTab = 0 Then
RichTextBox1.Clear()
For sss = 1 To 100
RichTextBox1.AppendText(" Tab 0 - " & sss & vbCrLf)
Next
ElseIf indexOfSelectedTab = 1 Then
RichTextBox1.Clear()
For sss = 1 To 100
RichTextBox1.AppendText(" Tab 1 - " & sss & vbCrLf)
Next
End If
End Sub
Private Sub Button2_Click_1(sender As Object, e As EventArgs) Handles Button2.Click
End Sub
End Class