The Basis shapekey is a shapekey containing your mesh without any deformation, the other shapekeys are your mesh modified from the Basis.
When you select a shapekey, you can replace the Basis field in order to use another shapekey instead of the Basis as a base for this shapekey.
Usually, you’re not supposed to have several Basis, there’s no problem for all your shapekeys to use the same Basis ; I guess you created them on purpose but I don’t understand why you did that, I think it’s not necessary.