Dropmods
Dropmods is short for "dropped craftmod". In general, craftmod are limited by the ability of the craftmod maker, up to 80 skills for the purpose of modifying an item, dropmods have no such limitation, and can have effective skills up to 150.
Dropmods can be appended to any item, as long as their intrinsic skill is high enough for the resulting item, instead of the 8 tiers from normal craftmod, their tier can be 1, 2 or 3. You can check the crafting skill the dropmod has by inspecting it with i #number in your inventory, like this: i 14. Each tier has a higher crafting skill ceiling than the previous, making T1 dropmod ideal for 5-slotting, T2 dropmod ideal for 6-slotting and finally the exceptionally rare and elusive T3 dropmod for 7-slotting. Since only T1, and T2 can be farmed efficiently and reliably, 7-slotting is usually not discussed by minmaxers in-game, unless the purpose is to 7-slot something using up to T2 dropmods only (which is possible if one chooses specifically "simple" modifiers, like jumbo pack as for example, but rarely more beneficial than using the best 6 modifiers on an item the game can offer)
Additionally, an item having a T2 dropmod will have all their stats increased by 5%, an item with a T3 dropmod will have all their stats increased by an additional 10%. These bonuses stack.