Usually these are rather arbitrary, but some cases are realistic, such as a hazmat suit's all parts. Individually the pieces provide various benefits but these do not literally add up to the full immunity to toxic materials a full set would. Or a full space suit giving the person ability to remain in vacuum for extended periods of time while only a part (even if there was a single piece missing) would make it near impossible.
Commonly the item sets are pieces of clothing or armor.
Sanguine # 2009-08-15 17:20:29