Your question and the answers are not exactly black and white, clear-cut. Facilities currently under Mandatory HACCP with USDA or FDA and facilities under voluntary HACCP organize their food safety programs differently and use various names for the different parts. It is a case of "a rose by any name would smell as sweet". People running food facilities use terms like Standard Operating Procedure (SOP), Pre-Requisite Program (PRP), Operational Pre-Requisite Program (o-PRP), Policy. Program, Plan, GMP, Curent Good Manufacturing Practice (c-GMP) etc. in an inconsistent manner with each other. Some want to simplify and others want to have tedious complexity. Here is my take on it:
HACCP Plans are sometimes conceptualized as a pyramid with GMP as the fundamental foundation. GMP documents generally are a policy statement of what your facility's GMP are going to be.
The next layer of documents are the Pre-Requisite Plans or Programs. These sometimes include programs like Shipping and Receiving, Blood Borne Pathogen Control, Intrusive Maintenance, Preventative Maintenance, Hold and Release, Hold and Test, Glass on Premises. In concept, if these PRP are under control then the frequency/severity of the risk/hazard is eliminated or significantly reduces to an acceptable level. PRP have varying elements but in general require certain features. Let me use Glass on Premises as an example.
Title: Glass, Brittle Plastic and Ceramic Program
Purpose: Why you find it needful to establish this particular program, e.g., To establish the Written program plans, policies, procedures and supporting documentation for the control of glass.....quarterly list(register) of known glass, storage, handling, training, disposal, acceptable and unacceptable types of plastic, plan for elimination of know glass
Scope: what facility(ies) this program applies to, e.g., ACME Food Company
Responsibility: what department, position titles and or individuals will be held responsible to follow, perform, monitor, document, verify, validate this program
Procedures: here is where your SOP enters the stage. Not all PRP programs require a procedure(s), but most do. In this case you need to write an exacting, detailed, step-by-step SOP for how glass will be stored, handled, registered, audited, cleaned up when it breaks.
Generally a procedure needs to address the who, what, when, where, how, why of the task.
It could be one big over-arching procedure or many smaller procedures to cover each element, e.g. a clean up procedure for cleaning up glass in a production area where food is exposed. Obviously it will need address the eventuality of glass getting into food.
stop production lines in nexus
report to QA/QC
Tape off the area to stop the spread of glass fragments
Collect exposed food from line into a box for so many feet from ground zero
Place X product on HOLD
Perform clean up X
have QA/QC inspect/verify
Shorter answer is that an SOP is a procedure directly associated with a PRP that requires exacting detail to control a risk/hazard.