Item usage determination

Hello all Gurus, SAP friends,

I get quick questions on item category determination and item usage.
1- how does SAP know that the item usage must be taken into consideration during the process ? is it in the SAP programs ? Company is using "interchangeability" (FFF class) and when a sales order is created for a material managed with FFF class (through PIC01 for instance) the system determines automatically PSEL as usage. How is it done ?
2- Here is In VOV4 (table V_T184) entries.
OR NORM PSEL                 TAD        TAPS
OR NORM PSEL                 TAN        TAPS
OR NORM PSEL                 TANN     TAPS
OR NORM PSEL                 TAW       TAPS
OR NORM PSEL                 TAX        TAPS     ZPRV
OR NORM PSEL                 ZAXN     TAPS
OR NORM PSEL                 ZTAH     TAPS
OR NORM PSEL                 ZTAN     TAPS
OR NORM PSEL                 ZTAX      TAPS
OR NORM PSEL                 ZTAY      TAPS
When a sales order is created for FFF material the system determines automatically the entry corresponding to TAX - TAPS (the 6th entry). Why does system pick this particular entry and not another one ?

I'll will be pleased if you could answer to these requests.
Thanks to all.
Rgds