Default Accounts in Procure to Pay process

 Hi Folks,

Could some one explain me the default accounts in Procure to Pay flow.

i.e When we create Requisition where system is picking Reserve for Encumbrance account., while booking PO where the Charge account is defaulting.

please explain the defaults for following accounts.

1. Receiving Inventory Account

2. AP Accrual Account

3. Expense Account

4. Encumbrance Account

and what are the updatable stages.