What is this redline?

[Favio]

This redline appears in the clip but isn't a automation It serves to adjust the volume think like envelope so I was restauring deleting nodes and moving it to 0db but in this last one I can't I deleted a node and now I can't drag it help ? 

I added and image check it

never know of this feature so I don't know how to activate it but I can use it because it was already activated 

[scook]

clip gain automation envelope?

Display for clips in this track was enabled when the edit filter was changed to this envelope. As you can see, the display is persistent.

The edit filter is this drop down

in the track header