generar letra del nif

[Beakdan]

Mientras yo modificaba una función que encontré, respondía Jarabas. Pero de cualquier modo, publico el código (nada del otro mundo). Sin embargo, yo aún no entiendo que se supone que son esas siglas ¿Pueden explicarme?

(Pega esto en un nuevo form)

Código:

Option Explicit
Private WithEvents m_Text1 As VB.TextBox
Private WithEvents m_Text2 As VB.TextBox
Private WithEvents m_Command1 As VB.CommandButton
'Encontré este código para delphi
'************************
'function NIF(DNI: String): Char;
' begin
'   Result := Copy('TRWAGMYFPDXBNJZSQVHLCKE',StrToInt(DNI) mod 23+1,1)[1];
' end;
'************************
'En VB esto es el equivalente
Private Function NIF(ByVal sDNI As String) As String
	NIF = Mid$("TRWAGMYFPDXBNJZSQVHLCKE", (CLng(sDNI) Mod 23) + 1, 1)
End Function
Private Sub m_Command1_Click()
	If m_Text1.Text <> "" And IsNumeric(m_Text1.Text) Then
		m_Text2.Text = NIF(m_Text1.Text)
	End If
End Sub
Private Sub Form_Load()
	Me.ScaleMode = vbPixels
	Set m_Text1 = Me.Controls.Add("VB.TextBox", "Text1", Me)
	With m_Text1
		.Move 10, 10, 200, 23
		.Visible = True
	End With
	
	Set m_Text2 = Me.Controls.Add("VB.TextBox", "Text2", Me)
	With m_Text2
		.Move 110, 39, 100, 23
		.Visible = True
	End With
	
	Set m_Command1 = Me.Controls.Add("VB.CommandButton", "Command1", Me)
	With m_Command1
		.Move 10, 39, 90, 23
		.Caption = "Calcular"
		.Visible = True
	End With
End Sub

Saludos.