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.
Sometimes used as just another term for 'linear mask', it offers a straight-line transition from an adjusted area to unadjusted. It’s usually a soft transition not a hard one, and you can vary the width of the transition zone. Linear masks are used most often for darkening skies in landscape shots, where they are like the digital equivalent of a graduated filter. A linear gradient can also be a painting tool for applying gradients (not adjustments) to images.