Storing of rows on AMPS based on UPI,NUSI,USI

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Storing of rows on AMPS based on UPI,NUSI,USI

Hi, 

   I am new to Teradata and self-learning Teradata . The manuals says many things about UPI,USI,NUSI.

 If a table has one PI,two or thre NUSI's , how is the data stored on AMP's? Is there any order of preference to consider PI and NUSI's of  a table? 

IF TD considers PI for distributing rows to AMP's, then what is the use of defining  NUSI/USI on other columns if rows are already distributed based on PI?

Sorry for a dumb question. 

Thanks

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