Aggregate Navigation is used to specify the incompatible 
objects.

Whenever we use Aggregate Aware we need to set up 
incompatilibities to work it properly (so BusinessObjects 
knows what SQL to generate)

For example, There are 2 aggregate tables fact table by 
week and by day.If you set up a measure as aggregate aware 
from both tables - total quantity for example, the select 
might be something like: - 

@Aggregate_Aware(sum(week_agg.qty),sum(day_agg.qty)) 

The week table listed first as this would be the 
preference. But as BusinessObjects needs a reason to not go 
to the week_agg table, this is done using 
incompatibilities. For example the Week Number dimension 
object would be compatible with the week_agg table as it 
will link to it and the data is held at an appropriate 
level to report against it. However the Date dimension 
object would be set as incompatible with the week_agg 
table, such that when it is selected alongside the Quantity 
object, BusinessObjects knows to jump to the next option in 
its select definition.