There are requirements to review the effectiveness of the system, including on-time delivery performance (as per 9.3.2). If there are no controls in place to make sure that planning, and understanding the organization's capacity, it may be hard to meet goal for on-time performance.
If the customer has an on-time delivery requirement (such as Jim suggested), the organization may find the need to proceduralize capacity planning.
In many ERP's systems, there is an emphasis on just that. To the OP, look there first if you are seeking your 'procedure'.