Rocket Validator looks for accesibility violations on your pages using the axe-core accessibility engine, hosted on our own servers and integrated into our web crawler. You don't need to buy an additional license, everything's already included in your Pro account at Rocket Validator.
WCAG: Best Practice: Frames must be tested with axe-core
Frames must be tested with axe-core.
Without the axe-core script, it is not possible for the tool to perform violation checking on multiple levels of nested iframes.