From BRWiki
A province's source potential is a measure of how many source holdings it can ever hold. The potential is restricted by the province's province level and the type of terrain it contains. Each terrain type has a specific magic potential. The sum of a province's level and the level of sources within it cannot normally exceed the magic potential rating of the terrain (see Table 7-1: Magic potential by terrain). The difference between a province's magic potential and its province level is its maximum source level.
See Also: BRCS definition of Source potential
