It should produce an inventory: identity and civil records, immigration history, evidence supporting the eligibility theory, and anything needed for family members. The deliverable is a sorted, gap-marked file, not a filing. USCIS issues its own instructions for Form I-140G supporting documents through the official portal after the application and fee, so build the inventory to be extended later.
Ask who holds and updates the file
Decide early whether records sit with the household or the adviser, how updates are tracked, and who is responsible for translations and certified copies. Agree that nothing is ordered in bulk before the official instructions identify what is wanted. Note which items expire or need reissuing if the process runs long.
A clear custody arrangement also matters if the household later changes adviser, because the organized file should remain usable by whoever takes over. Ask for the inventory to be organised by what each item is meant to prove rather than by document type, since that is what makes it extendable later. Identity and civil records, immigration and travel history, the material supporting the eligibility theory, and each family member's own records are four different groups answering four different questions.
Mark every gap with what would fill it and who can obtain it, and agree that nothing is ordered in bulk before the official instructions identify what is actually wanted, since documents ordered early can age out before they are used.