It's a percentage. Per the MEL it would apply equally to distances calculated on wet or dry runways. In the TransAsia accident Airbus provided calculations for an exhaustive variety of combinations of thrust, braking, spoiler deployment and runway condition. All the numbers can be found in the accident report.
ELAC
PS - The specific value (e.g. 1.15) seems to vary a bit depending on mod status. For a previous carrier's A330's I found numbers of 1.15, 1.18 and 1.2 applying to different aircraft in the fleet.