The scale of 0 to 120 is a percentage value. At 100% the amount of correction should in theory be exactly enough to equalize brightness across the entire field of view if the photograph was taken of a uniformly bright field.

Depending on the particular lens in question, the 'compact' lens profile loaded into the camera at the time the image is shot may or may not be detailed enough to actually accomplish this at every focal length and aperture combination. When any Canon raw image file is first opened in Digital Photo Professional the in-camera settings used when the photo was taken are applied by default (unless the user has changed that setting in DPP). So whatever values were used by the camera are what DPP starts with. Even if Peripheral Illumination Correction was turned on in the camera when the photo was taken, if the camera did not have a profile loaded for the specific lens used, the default value will probably be '0'.

Sometimes a photographer may want some light fall-off in the corners but not as much as what the lens gives without any correction, so the option is there to adjust the value anywhere from no correction (0) to full correction (100) to more than full correction (120).

One can presume that the 120% setting is available for cases where the entire field of view of the scene was not uniformly illuminated when the photo was taken but the photographer wants to make it appear uniform in the end result. One example would be a wide angle photo taken using the camera's popup flash which tends to be brighter at the center and dimmer on the edges and in the corners.