The iFrame Control module authorizes domains to be used in iframes within the system.
If you want to allow your users to use iframes 2 things must happen. First profiles that are allowed to use iframes must have 'iframe' added to their "Allowed HTML Tags" setting for the quota that they are in.
And second that the domain they use must be authorized by the iFrame Control module.
Only domains listed in the iframe control module are allowed to be embeded.
Allowed iFrame Domains
When you want to allow a domain to provide content via iframe, first that domain must be authorized by adding it to the Allowed Domains.
For example if one of your users wanted to add a google calendar, then google would provide the code: