When making a complex part that is required to be stiff but as light as possible the type and density of the infill pattern is important and needs to be different for the different shapes within the part.
To achieve this I had to make it in two parts with different infill patterns and then glue them together although I could just physically print them as one on the printer,
These are the two parts joined
As part of an RC plane it is rather a specialised application but being able to use different infill patterns on different same object would be useful..