If you allow iframe embeds, you’ll be able to use the embed on Medium and other websites that don’t allow
<script> tags in posts.
The downside is that we can’t detect screenshots through an iframe. If someone tries to take a screenshot, the watermark will not appear. However, the credits will still be accessible, and the watermark will appear when dragging or saving the image to your desktop.
If you don’t allow iframe embeds, your images and credits will still work, but the watermark will always be present.
Images will still follow all url whitelisting rules you’ve set up for the image, regardless of this setting.
Normal(non-iframe) embeds will not be affected by this setting, and will still protect against screenshots.
Examples of all the scenarios can be seen here.