I'm sorry to disagree with some others here, but flash guide numbers do not have units - it is a just a number. This point was drummed into us in my first formal photography class. The guide number uses distance to determine an aperture as a starting point for exposure. Because the guide number uses distance to compute aperture, it is necessary to have a different guide number for a different unit of measurement. Most of the time a flash will list two guide numbers - one for distance measured in meters and one for distance measured in feet. Unfortunately, many manuals/instructions list the numbers something like "12 meters/30 feet" and so it has become a common misunderstanding that the guide number itself is expressed in units.
So... When the manual shows "Guide Number 12/39 flash (ISO 100, m/ft)" they are saying: 1. If you measure the distance in meters the guide number is 12; or 2. If you measure the distance in feet the guide number is 39.