External Users Access

I am creating a vault for customers so they can send us documents through a vault that is marked for external customers.

If a user is marked 'external' will the user be able to save documents to the vault?

It says here that "External Users cannot see or access any documents other than those specifically marked for them." 

Creating a User (m-files.com)

So if customers cannot see what is in the vault, will they at least be able to save to the vault?

Can I mark the permission to define the class as accessible to external users?