Re: SOP, Work Instruction, & Procedures
It is a matter of tiers, document hierarchy and level of detail. It looks like an organizational chart.
SOP, Standard Operating Plan if I understand you right, would be the top layer, or first tier: it is more strategic, overall addresses the program or organization, and has little detail. Of the various documents, this one is most likely to be circulated to customers; if so it is usually devoid of confidential details.
Procedures (second tier) describe the process. They are more detailed than the SOP, more formal than work instructions and will get a large amount of scrutiny during audits. They might have flow charts or point to more documents to support or give even greater detail.
Work instructions (third tier) are the lowest of these layers: closest to the actual work being performed, they might have step-by-step operational instructions or visuals; they might be based on a user's manual or they could be written by someone skilled in doing the work. There may be many work activities or portions to a process; the work instruction is usually dedicated to, written specifically for one of them.