Which game are you asking about?
In SE5, its right at the top of SystemTypes.txt. The percentages are used for generating all planet values of "Any". Manually specified planet types, sizes, and atmospheres do not get taken into account. In the stock game, there are very few such planets though (a few Huge planets with tiny/none moons), so the balance is fairly accurate.
If this was meant for the SE4 forum, you can use
this program to see type distributions for any given data file set.