Multi Video inside 1 Layer.

Hi. I have a question about the way how work the layer.
I mean when I trigger a video with the Sapphire media on the layer 1 for exemple. The media on the roller cue become L1. If I trigger another media it become L1+ or something like that.
What does it means, how it work?
And after triggering lots of media if I pull down all the fader on the desk some media keeps running. How can I do to have a black instead of having media running when all the faders are down?

Thank you for your help.