Blackmagic Design DaVinci Resolve Advanced Panel - Manual (Page 214)

COLOR 214
4. Drag a connection line from the second Key output of the EXT MATTE node to the
Alpha Output bar.
You should now see a successful composite, with the external matte creating a region of transparency
in the foreground clip though which the background clip shows through.
Before and after the final composite, created by connecting an
Ext Matte node to the Alpha Output in the Node Graph
External Matte Support