Question about storage type 902

Hello Everybody!

I´m implementing SAP WM and I have a doubt and is the next one:

When I do a material movement, system returns an error if the material is not create in the destiny storage type.

On the other hand, I create a purchase order and when I make the good entrance, the system stocks the material in the storage type 902 even the material is not create in this storage type.

Why is the reason for this? Is not necessary to create the materials in the standar storage type like 902?

Thank you very much for your help!