Hold a color negative up to the light and the whole strip looks orange. That is the orange mask - not dirt or age, but part of the film's design. Color dyes are inherently imperfect, so manufacturers build an orange corrective layer into the emulsion. In the darkroom era this mask is what made accurate color printing possible.
When digitizing, simply inverting an orange-tinted negative produces a heavy blue-cyan cast. Scanning therefore has to measure and cancel the mask first, then invert the tones to recover the real color balance - this step is what "mask removal" means.
Our scans measure the base color of every roll individually, remove it, and then fine-tune the palette to the character of each film stock. The same negative can look dramatically different depending on how well the mask removal is done.