There are a few different type of mods. You have rural mods, building mods and most extensive the gear mods.
The building modifiers are a way of boosting the power in each building. The building boost goes to level 30 as of July 2017, but it’s pretty clear that this will increase with time. Specially now that MS launched level 35 and 40 in their building upgrades.
The building mods are based upon some sort of generator that gives different outcomes each time you press the button. In addition it shifts the boost capability of the building between attack, defense and shielding.
The level keeps raising slow until level 25-27, then the volume of modifiers for each level increases significantly.
Mods used in gear are in general difficult to get hold of. You get them if you attack rebel targets, or if you win a battle, but most mods comes in crates when you buy packages.
The best non combat mod level is 6. So all mods for Base operations are level 6.
Combat mods are different, and the highest so far is level 22.
Gear mods for reseach
The most common mods you can use in order to improve the research boost and thus reduce the research time are these 4.
Gear mods for Construction
Some of the mods are giving boost in more than one category. There are a few mods like this, and a few of those gives boost in both Construction and Research.
In cases where you do not have enough mods, and you need to reuse them, the mod preset can save some time each time you change gear.
The most used Construction mods are these:
Gear mods for training
The mods that are most used for training gear are the following: