An item category is a multi-segment value that describes the item. An example would be: Corporate.Professional Services.Consulting.Software This is also a reporting tool and depends on the client's entire purchasing strategy.
category is a logical classification of items that have similar characteristics.
The catalog is a grouping/collection of items from one supplier that is used primarily in iProcurement for creating requisitions. A catalog can be internal (sourced from a Blanket PO usually) or external, via a punchout in iProcurement to a supplier site like Staples, Office Depot, Corporate Express, etc.
The values of those descriptive elements for each item assigned to the catalog group are stored inMTL_DESCR_ELEMENT_VALUES. Recommended aliases and recommended categories to be used with the items assigned to a catalog group are stored in MTL_ICG_ALIASES and MTL_ICG_CATEGORIES.