When you add source, it includes both source front and source matte input. When you add a source front only, a black matte will automatically be created.
To add source node, front source and matte source:
In the Sources list, the source is added. In the schematic, the process tree is updated: a source node with the selected clips is connected to the Paint node.
The source node is added and connected to the Paint node.
(a) Source node (b) Paint node