Muhammad,
Good catch! As for WRX, that's the GR/IR account. It has to be defined in config. For a quick definition: http://scn.sap.com/thread/2020185
As for where to set it up, there are connections in the configuration of the material master to valuation class. You set up the account determination in SPRO, under Materials Management - Valuation and Account Assignment - Account Determination - Account Determination without Wizard - Configure Automatic Postings.
For details, use Help on SCN. I imagine that someone has made a very nice explanation.
- Jeff