A matte key allows you to create a text effect in which the text's fill is, in fact, a background clip. When used with an Axis effect (or a Sparks effect that takes a matte), it can be used to layer text made of one background on top of another background.
All static (non-animated) fill colours are set to a transparency of 0%.
In the following example, the front layer is used for the text’s fill. The fill is then composited over the background layer with an Axis Key soft effect.
Image courtesy of Technicolor (formerly Toybox)
The Alpha Processing Mode box changes to RGBA and the Fill field resets to 0%.
The text fill is composited over the background layer. This text layer is then applied to the timeline background layer.
Image courtesy of Quietman, Technicolor (formerly Toybox)