A data island is a table that is not connected to the main data model. It is often used when you want a UI element that users can select on but not directly filter the data model. For example, if you want the users to pick a metric and then change the metric in a chart.
From the reference manual:
When data that includes circular references is loaded into QlikView, loosely coupled tables are created automatically, to avoid that the circular references create a loop in the QlikView internal logic. These loosely coupled tables need to be handled in order to visualize data in a way that is expected and understandable. See Circular References for further information.
Any table can also be made loosely coupled interactively from this dialog or via macros. Additionally, it is possible to declare loosely coupled tables explicitly in the script via the Loosen Table statement.
The normal QlikView associative logic is disconnected internally for loosely coupled tables. This means that selections in one field do not propagate through to the other fields in the table. This may be very useful in a number of situations, typically when it is needed to avoid circular references in the data structure. For more examples of how this feature can be used, please refer to the appendix Intentionally Creating Loosely Coupled Tables in the QlikView Reference Manual.
The major difference is that a selection in a "regular" table linked with the loosen table should make selections in the latter, but not the other way around.
I recommend avoiding using loosely coupled tables. The behavior of this tables changed multiple times from one QV version to another, so an application that works today may be not working tomorrow.
