In the "old" days we had procedures that covered the ISO standard's requirements in full but not in every detail. For example, document control required that documents be reviewed and approved by authorized functions who had access to pertinent background information on which to base their decisions. So the procedures, we called then level 2 because the manual was level 1, would pretty much echo what the standard required and would say something like "Documents are reviewed and approved by authorized functions who have access to pertinent information on which to base their decisions". The procedure might say that the Management Rep. for the QMS approves all level 2 procedures, departmental managers approve all level 3 work instructions, etc. But the details, like how they approve, electronically in a database with workflow capabilities, on paper with a serial routing, etc., the details, would be in the work instruction. So that is why work instructions change more frequently than do procedures and hat is also why they are used more ... they are where the "rubber hits the road".