Extensible Markup Language (XML) is a common format for storing and displaying complex datasets. XML has a set of rules for encoding documents or data in a format that is both human-readable and machine-readable. XML has a similar structure and usage to JSON. Ignition uses XML in a number of places behind the scenes, most notably when displaying Report data or storing Vision windows.
An X.509 certificate is a digital certificate that uses the widely accepted international X.509 public key infrastructure standard, which verifies that a public key belongs to the user, computer, or service identity contained within the certificate. In Ignition, you will see references to X.509 when setting up JSON web keys for OPs.