Material Overrides allow you to edit materials inside complex/deep mesh structures like SVGs or Text. This can be achieved via the Sub-Mesh Behaviour's Fill or Stroke tabs.
When you add an Alpha Material Override to a Sub Mesh you can blend between the original alpha and the one you set. You can then, for example, connect a Falloff to the Sub-Mesh to control the effect.
Alpha - set a value for the alpha.