Dynamic Lightbox items are string values that are passed to a lightbox via a url or by accessing existing content on a webpage with JavaScript. Dynamic items can be added via the Web Form's administrative interface. Read more about Dynamic Items.

When setting up a dynamic item, you can add a fallback value:

The fallback value will be displayed whenever the conditions for the target fill value are not satisfied. 

When setting a dynamic item to target a text block, the text in the editor will act as a fallback if no fallback is set in the dynamic item.

If you have multiple dynamic items set up using the same target, you only need to set a fallback value for the last dynamic item in the list targeting that value.