"Frags"

At higher lvls, reconfiging parts with regular armor parts found from killing mobs/bosses is not a good option anymore. And generally speaking, ranking up parts past rank 14 is hell (they need over 120K reconfig exp). Thus the need for frags. There are 3 types of these.
The most common one provides 25 reconfiguration experience points, and you can stack up to 40 of these in 1 inventory slot (which means 1k reconfig exp per slot). These can only be obtained from the lvl 15+ instance (Exotic Planet) in large amounts (around 22 stacks per trip), and from the lvl 18+ instance Space Base, but in much smaller amounts (with a lot of luck, 5 stacks or so per trip).
The next most common one provides 100 reconfig exp points, and can be stacked in groups of 10 per inv slot (again, 1k reconfig exp). These can be obtained from bosses in Exotic Planet (random), from supply shops (100k Godas each), as bonuses for filling up experience bars (random), and from kryogen and higher lvl mines on Armor Battle and Space Wars servers. On average, these mines may give 5-7k reconfig exp if you solo. They can also be obtained from the Gold Mines in Ice Planet, which are located past the Element Digging Valley map. The mines remain up for 2 hours 5 mins at most, and throughout that time, any char should get around 60 stacks + ~3m worth of crystals.
The last frag type provides 10k reconfig exp, and can only be obtained from supply shops (10M each) or as Death Match prizes. These can be stacked in groups of 100 (credit for that info goes to Myr).

Due to the important role that reconfiging plays in this game, for a long time the ingame currency's value was basically lower than that of frags. Players would sell frags to each other, the price usually 350-400k per each 1k reconfig exp, or trade them for rare weapons, body parts, and cash shop items. After an update made frags became available for purchase in the Cash Shop, their use as "currency" has diminished. They are also useless for armor combining, since combined armors can only be ranked through Cash Shop Nemo Crystals.