Caution while assigning the approval group in the approval assignments window ?

Hello,

While assigning approval group to a position in approval assignments window, i am getting the below caution, when i hit the save button.

What is meaning of this message and what is oracle trying to say.

'Please include the required approval rule in your approval group for this Blanket Release..'

We already have a approval group assigned to the document types blanket purchase agreement and blanket release.

I am getting this error while i am trying to assign an approval group to the document type planned purchase order.

--- Thanks,

Emma