When Binning is set to Set on the Dimension Settings tab of the Decision cube editor and Start Value is not NULL, the dimension is said to be "paged," or "permanently drilled down." You can access data for just a single value of that dimension at a time, although you can programmatically access a series of values sequentially. Such a dimension may not be pivoted or opened. 

It is extremely memory intensive to include dimensional data for dimensions that have very large numbers of values. By making such dimensions paged, you can display summary information for one value at a time. Information is usually easier to read when displayed this way, and memory consumption is much easier to manage.