... while a Flight Manual refers ..
The flight manual is the document required by the Design Standards to be produced and provided, eg for FAR23 aircraft, the present requirement appears
here.
The POH style of document originated with GAMA (
Specification No. 1) and ICAO (Guidance on the Preparation of a Pilot's Operating Handbook for Light Aeroplanes - Doc 9516) documents.
SOPs are found in an operator's Operations Manual, or similar document ...