Collections

A Collection is a set of 5 items dropped from a specific enemy unit type (though new units sometimes also drop these items) which can be traded in for certain rewards, with 1 of each traded in for the given reward.
There are eight Collections in the game, which are in order:
#Scout Collection
#Infantry Collection
#APC Collection
#Artillery Collection
#Battle Tank Collection
#Stormtrooper Collection
#Rocket Battery Collection
#Versus mode collection
==Gold==
Besides defeating enemies or being sent gifts, individual items in the collections can also be purchased for Army Gold (as opposed to Cash):
#The Scout and Versus items cost 1 Army Gold each.
#The Infantry and APC items cost 2 Army Gold each.
#The Artillery and Stormtrooper items cost 3 Army Gold each.
#The Battle Tank and Rocket Battery items cost 4 Army Gold each.