An additional modeling convention is to add unique identifiers for each attribute and fact. Unique identifiers denote the key that maps an attribute to its source data in the source system, when applicable. This information can help define primary keys in the physical warehouse schema (see Uniquely identifying data in tables with key structures).
Remember that facts are usually identified by multiple attributes and therefore will have multiple unique identifiers. The following diagram shows a logical data model with unique identifiers added. Some attributes rely on more than one ID column to identify its elements. For example, note the Item attribute, which requires both the Item_ID and Class_ID columns to uniquely identify its elements.