How it works...

Defining data source privacy levels such as Organizational or Private prevents the data from these sources being exposed to external or inappropriate data sources during data retrieval processes. For example, if a query calls for merging a Private data source with a Public data source, the join operation will be executed locally--the private data will not be sent to the public source. In the absence of Privacy Level settings set for data sources, the M query engine will look to optimize for performance by utilizing source system resources.  

The options under Preview features change with new versions as some previous options become generally available and new preview features are introduced. The monthly Power BI Desktop update video and blog post provides details and examples of these new features. Usually a restart of the Power BI Desktop application is required once a new preview option has been activated, and tooltips and dialogs in Power BI Desktop will advise you if a preview feature is being used.

The enable tracing option in the Diagnostic Options section writes out detailed trace event data to the local hard drive and thus should only be activated for complex troubleshooting scenarios.