Often when you do a job you gain objects, one type of this objects is for the "Collections". Completing one of them grants you a special item, usually more powerful of which found in the store or never selled by it. There's 3 different collections:
1) Area (items recovered only in one of the different areas of the game);
2) Buildings (items recovered only from this buildin no matter if is in area or in a rival hood);
3) Fight (items recovered when you fight other players in the fighting arenas)
is possible to buy the special item through FB Credits.