Information density indicates the percentage of rows that contain a distinct value for a field.
Subset ratio (only on key) shows the percentage of all distinct values for a field in the table compared to all the distinct values for that field in the entire data model.
Both indicators are paramount when verifying the data model. They indicate if the key chosen is a quality field between 2 tables. If there are no/little common values between the 2 tables, it means that the key cannot be used or that the key is not precise enough. It saves a lot of time and can be used to check a key before joining 2 tables.