Items within a dimension may correspond to real world objects, e.g. a branch of a business or a product, but may also be more conceptual classifications, such as purchase type. If the dimension is scoped to the trading partner level, then dimension items will be organised under their respective trading partner within the admin area.
By default, all dimension items have a name, reference, start date and end date. Setting the dates for a given dimension item will control whether it is available for selection against a given program line. For a given start and end date, a program line will only see dimension items after the start date and before the end date. If no dates are configured, then the item will always be available, subject to other filters.
More information on dimension items is found here.