Storing of rows on AMPS based on UPI,NUSI,USI - forum topic by TeradataMaddy

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