The descriptor of an FX is all at once extra color and description for an FX and a vital component of the FX as far as the rules of the game are concerned. Most of the time, the descriptor is the former: a character who has Close RangeDamage 5 with the Fire descriptor and a character who has Close Range Damage 5 with the Electricity descriptor will be doing largely the same things with these two FX for most of the game. The players and GM will describe their results differently, of course, but the results will be the same mechanically.
Once in awhile, however, the differences in descriptors matters. For FX like Boost, Immunity, and Nullify, the descriptors on FX matter. For instance, a Nullify FX which can nullify Fire FX can be used to counter the character with the Fire descriptor on the Damage FX, but not the one using the Electricity descriptor.
A descriptor can be used to describe many different aspects of an FX, but how much time you need to devote to descriptors depends entirely on the game. In a fantasy game, for example, the “Magic” descriptor might be sufficient for most of a wizard's spells. But if there are also spellcasting priests, then it might be helpful to further distinguish between “Arcane” and “Divine” when it comes to magic. And if you specialize further still by creating an ice mage, then you might decide that it's best to have “Arcane Ice Magic”.