'Release Blanket' heirarchy ?

Hello,

We have 3 different position hierarchies in use for various document types.

1) Heirarchy 1 for purchase requisition

2) Heirarchy 2 for Standard Purchase Orders

3) Heirarchy 3 for Blanket Purchase order

Now given that for the document type 'Release Blanket' the default heirarchy is blank, which hierarchy does blanket releases follow ?

Is it heirarchy1,2, or 3 ?

----Emma