Not being your customer, and having done only a small amount of work with aerospace suppliers I don't know for sure. I could see it as:
- a manual (e.g., set of procedures, forms, etc.) that spells out the processes used for developing and validating designs (and design changes), basically equivalent to section 7.3 in ISO 9001 (or AS 9100).
- a set of guidelines used for designing specific products (e.g., formulas to use, minimum &/or maximum limits of capability, test methods).
Best thing to do would be to ask your customer what they mean by it.