The Filetransfer appliance Dropbox XML API is a function to automate sending files through a dropbox. This feature was actually born out of the need to send support information from Filetransfer appliances when requested.
The easiest way to explain is probably with an example script:
#!/bin/sh
filetransfer="https://fta.example.com"
api_key="abcdefgh1234567890"
attachment_id=`curl -X POST -F Filedata=@bigfile.zip -F api_key=$api_key $filetransfer/attachments`
cat <<EOF | curl -s -X POST -H 'Content-Type: text/xml' -d @- $filetransfer/dropbox/api
<?xml version="1.0" encoding="UTF-8"?>
<message>
<api_key>$api_key</api_key>
<subject>Subject</subject>
<message>Please let me know what you think!</message>
<attachments type='array'>
<attachment>$attachment_id</attachment>
</attachments>
</message>
EOF
The api_key comes from the Admin → Dropbox configuration. It's a unique key for each dropbox. As you can see, it's the only identifier that's needed to send files.
The purpose of the api key is to provide some protection. On the web interface, there's other protections like authorization tokens that prevents someone from automating an attack (the authorization token needs to be fed back to the server.