If your clients need a visa for travel you can store that on their contact record. Currently, Tern does not have a dedicated visa section, so we'd suggest one of the following:
Upload a copy of the Visa to the documents tab - This is ideal if you have a PDF or similar copy of the visa itself. This also is helpful if you can print out a confirmation of the Visa to a PDF.
Add the key details to note - This is a great way to do it if you want to keep track of specific information since it's most flexible. You can create a new note with the title "Visa for {{Country}}" and enter in the key details there.