How come the G/L will automatically showed during PR/PO creation with account assignment category assigned. e.g. K

[@more@]Just imagine that, if you create a Purchase Order/Requisition without an account assignment.. this will lead to the system using your stock account automatically through:
Material Master --> Valuation Class --> OBYC --> BSX (generally)

Now..
If you choose to use an account assignment.. there are 3 scenarios...lets assume we are using K (Cost Centre)
1) Material Code with Accounting View --> Valuation Class
2) Material Code without Accounting View --> NO Valuation Class
3) Short text purchases

Scenario 1
System will search for Material Master --> Valuation Class
Then, OBYC --> GBB --> Gl Account for VBR for Valuation Class above.
Why did i say VBR? In your system it might be different. Check from SPRO --> MM --> Purchasing --> Account Assignment --> Maintain Account Assignment --> K Cost Centre --> Acct modification

Scenario 2
System will ignore Material Master --> Valuation Class
Then, OBYC --> GBB --> VBR --> GL Account with Blank Valuation Class

Scenario 3
Same as Scenario 2

As long as you choose to use an account assignment, system will automatically search in GBB... but where it searches within the GBB, is based on your configuration in Account Assignment --> Acct modification..