Most tutorials on this site have been designed to build on top on others.
This makes it easier to follow them in specific sequences.
A series represents this kind of sequence that has been premade with a specific kind of output and topics in mind.
A series is like a playlist.
This video shows you how to create Custom channels for 3D compositing in Nuke.
Whether you want to do your own Custom-pass Compositing or use mattes for different objects you have rendered in Nuke, this tutorial has everything you need.