Coins are used for many things. You can use coins for buying things at the auction house, perm items such as +0 rings and bracelets, perm weapons(any weapons),buy items in the shop, renew a stage when you fail, jewel farm event and such more!
Vouchers can be used for things but not many.You can use vouchers for buy items in shop that are vouchers and you can perm(30days), buy rings and bracelet all atri +5 they are permeable tho, you can buy fortune wheels, and angel jars
Gold can be used in different ways. You can covert weapons meaning switching the lvl and atri+pearls to another weapon(10k gold for it), You can go to the spa~ 10k gold for like a hr i think or so~,
[color=cyan]Medals are really great but is a pain in the butt to get~ , medals are equivalent to coins except you dont buy them you earn them, You could buy energy stones(lvl 4 are 7 and lvl 5 are 30)You can buy **weapons which i think are 340 medals but is 365 days, you can buy sync stones lvl 4 are 3 and lvl 5 are 15 medals, You can buy outfits there and some are 340 others are 680 or so but they last for the year. You can also buy exp pills lvl 1 is 2 lvl 3 are 4