Overriding level key
You can join a dimension table to a fact table based on something other than the level key columns for a specific source. This feature overrides joins between a given dimension level and fact tables.
For example, if a given level has a composite level key defined as column1+column2, the user-ready data store will be loaded with the composite level key configuration. But after the data store is loaded, while joining the dimension level with that fact, if you only want to use column1, you would define the override level key as just column1.
You can only override a hierarchy level's keys by using a subset of that level's keys.