From the Book QlikView 11 for Developers:
"The name of the field. Optionally, if the field is a key field, a qualifier is shown enclosed in square brackets. This qualifier indicates the following levels of key quality:
[Perfect Key] indicates that every row contains a key value, and that all of these key values are unique. At the same time, the field's subset ratio is 100 percent. This qualifier should be seen in dimension tables, where every key should uniquely identify a single record.
[Primary Key] indicates that all key values are unique, but not every row contains a key value or the field's subset ratio is less than 100 percent
[Key] indicates that the key is not unique. This qualifier is usually seen in fact tables, where the same dimension value may be associated with many different facts."