They are both designed for slightly different purposes. ModuleSettings are unique to a particular ModuleID, that is when you place a new Reports module on the page (aka tab), it has a unique ModuleID and its own ModuleSettings. However, you can also place a copy of an existing Reports module from one page (aka tab) onto another page. This copy has the same ModuleID and shares its ModuleSettings with the other copy, but has a unique TabModuleID and its own TabModuleSettings. So basically, the idea is that the Data Source Settings are common to all copies of the same Reports Module, whereas the Display Settings can be different for each copy.

When developing your own module, you need to evaluate at which level the settings should go. My general rule of thumb is: If the setting has no effect on data access or manipulation and is purely a UI setting, it should probably be a TabModuleSetting, but if it must be shared among all copies of the same Module instance (for example, it affects the data retrieved/stored) then it should be a ModuleSetting.

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Andrew Nurse
DotNetNuke Core Team Member and Reports Module Project Lead
Microsoft Certified Professional Developer