Data Islands can be created from any data source and are disconnected from other tables in data model.
For ex. in data model let say we have 10 table, out of which 6 tables are interconnected to each other and rest 4 are interconnected to each other. But these group of 6 and 4 tables are not connected to each other. So there are said to be 2 data islands in data model.
From time to time, you may want to load data that is a separate table, disconnected from any other field in your data model. This is called a data island. Data islands can be created from any data source and are often created as an inline table. Data islands can be used to reference dates, table headers, or whatever you need to be disconnected from most of the existing data in the data model.
One useful example of data islands is to aid in the construction of table charts in the presentation layer of your document, for instance, to allow you to create rows containing measured values rather than dimensions. Let's use a call center example where we track customer representative telephone calls and call times.
What is he correct at? If you mean - there is no such use of data island, sorry, you both are NOT correct.
There would be situations when you would want one segment of your reports to be unresponsive to any selection to a different set of filters, because they are logically unassociated. Then you have to keep the model disconnected resulting an island. Sometimes developers loads some fields that is used to handle trivial issues like setting toggles between views or so; setting unassociated flags...all would result islands and those are useful. I could give many such examples, but I am lazy.
A typical use of date islands is in User Interface-design and Ad-hoc reporting.
Imagine that you have a straight/Pivot table with a large number of interesting expression columns. The table may be too large for most users, and different users may be only interested in different sets of expressions. How do you make the layout of this table configurable?
Create an unconnected INLINE table with a single field containing unique names for all expressions. Present this field as a list box on the sheet of the large table. For every expression, add a Conditional show expression that makes the expression column visible whenever the name of this particular expression is selected in the list box (for practical examples, search the community).