file format to transmit to third party

what is the recommended file type to send for example to a customer? I believe an assembly can be sectioned and measured if I use a step file. I would like to be able to give a model to a customer to insert my device into his overall assembly but do it in a way he can not section it to take internal measurements. Any suggestions?

You could save your assembly as a part file, and chose "Exterior faces" as a save option. You could then export this part file in any format you like.

You customer could create a section view of the part, but it would appear hollow.