What does it need to fit?
Identify the mating parts and the features that control the fit: mounting holes, slots, seating faces or clearances. Include the dimensions and units rather than relying on the file name to explain the intended scale.
For a cover, guide or spacer, highlight the surfaces that meet another component and explain the clearance needed.
What will it experience?
Describe loading, movement, handling and exposure to heat, moisture or sunlight. Mention requirements that cannot be compromised and distinguish them from preferences.
Material family, grade, geometry and print orientation all belong in the discussion. No generic material description establishes suitability for every application.
Agree the important details.
Mark the visible faces and any areas where surface finish affects assembly or use. If the design is still developing, a prototype may help answer fit questions before you request a run.
If your application needs documented performance or a particular standard, include that requirement in the brief.