Variable Name for a file to used in the Packer configuration JSON.
For certain builds, like amazon-instance the x509_cert_path & x509_key_path are required. The given variable name can then be referenced in the configuration file as the value for a path.
Example JSON configuration:
{ "type": "amazon-instance", "x509_cert_path": "{{user `x509_cert`}}", ....etc... }
Then the Variable Name would be: x509_cert with the contents as the cert. Others such as x509_key_path can be specified in the same manor.
For more details consult Packer documentation.