Item series is a generic term for broad level classification of the items. For different type of items the purpose of the item series changes to some extent.
For finished goods item series is Business Division of the products. Typically it is mapped with marketing team which is responsible for promoting the products. Sometime it can refer to line of business.
For materials it is generally classified as Raw Material, Packing Material, Engineering Items, Semi Finished Goods.
Fixed Assets has separate item series. Some organization prefer to maintain separate series for different fixed asset sections.