"Additional documentation" refers to extra written information or proof that is required or requested in addition to what has already been provided. It is needed to further support or clarify a situation, process, or claim. Full definition
Some countries will require additional documentation for families travelling with children under the age of 18. (somak.com)
You may be asked to provide additional documentation such as bank statements showing payments or cancelled checks to support your dispute. (valuepenguin.com)
They may ask for additional documentation from your school to verify that you are attending or plan to attend that school. (studentloans.net)