Besides lettered mods, there are also simply different mods for every stat.

An example is Cunning: there are both Patron mods and Skill mods. Patron mods are heavier on Endurance, while Skill mods have higher Cunning. Or Aim: there are both Commando mods and Reflex mods (same difference, respectively, as above).

This dichotomy exists with every stat. Presumably, you would want to load the higher Endurance mods if you were running a tank. However, not every type of mod is available from the Planetary Commendations vendors at every level. This is part of the strategy of when to upgrade.