Specify material and construction.
Request the proposed material specification for the bottle, lid components and any internal parts. If a particular grade or performance is important, name it in the brief and ask how it will be documented for the production order.
Describe how the bottle will be used.
Daily hydration, outdoor use, school use and corporate gifts may lead to different decisions on capacity, weight, mouth opening, carrying features and packaging. The intended user is a practical part of the specification.
Check the closure system.
The lid is often where usability and leakage risks are concentrated. Ask for a clear description of the lid assembly, seal components, cleaning requirements and any replaceable parts. Test sample lids repeatedly rather than checking only initial fit.