Stock type generation logic in PO creation

Dear All,

I would like to ask about the stock type generation logic while creation the PO.

the following is the background.

Our system status:

1. The QM module is activated.

2. the "post to quality status" field is NOT ticked in material master.

Right now,

1. when I create the inter-company sto, the stock type is defaulted as "X", QI stock.

2. when I create the intra-company sto, the stock type is defaulted as blank, U stock.

does anybody know what cause the difference of two po?

Thanks very much in advance!