FileList之問題(編號:3496)

Private Sub FileList_MouseDown(Button As Integer,
Shift As Integer, X As Single, Y As Single)
Dim DY As Integer
DY = TextHeight("A")
DragLabel.Move FileList.Left, FileList.Top _
+ Y - DY / 2, FileList.Width, DY
DragLabel.Drag
End Sub
其中的TextHeight("A")代表甚麼呢?
   又可將括號裡的"A"換成別的嗎?
   為甚麼呢?