These Control Lines can already be found in DxO PhotoLab 6, but they are new to the Nik Collection 6. Control Lines are a special kind of selective masking tool. They work like a linear gradient tool, but with an eyedropper which can be moved around to select the tones or areas you want the mask to be applied to.
Control Line (DxO)
This is a new type of local adjustment tool introduced by DxO into its PhotoLab and Nik Collection software. You can think of it as being like a linear gradient tool but with an eyedropper to select the color and tonal range you want the adjustment to target.