For Tableau to understand your data it has to be in well structured format meaning all your dimensions and measures has to be in tabular format. Tableau cannot understand unstructured data.
so if you're integrating obiee database or any other database with tableau, so it has to be first converted into a tabular form right??
If Tableau is integrated with any database say Oracle, MySQL, etc. if data is in a structured format then definitely Tableau understands them and classifies as dimension and measure based upon the values stored.
Also one devoted to just Excel:
Lots of threads about preparing data out there on the forum too, cheers and best of luck!