First answer of this post will lead you here, where you’ll “define” your custom channels.
Then you’ll be able to use them under collision presets in blueprints;
For the C++ part, If you look at the DefaultEngine.ini, you’ll se something like this:
ECC_GameTraceChannel::ECC_GameTraceChannel1/2/3/… are surpluses the engine leaves to you to define as you want.
You can even sugarcoat them to your liking;
And use them like this;