Multi-Select Callback

Hi there,

What I want to do is to populate one multi select field with the selected values of another multi select field. I’ve read that I can add a callback function to a multi select field in order to populate it’s values. I’m refering to the release notes here:

Adds new select/multi-select options to retrieve options from a callback.

In the form builder, edit or add a select/multi-select field and add your callback function in Choices callback field to get populated.

My question: What’s a callback function? Is it just a normal PHP function or some of WP add_filter or add_action or something completely different?

Do you have an example how such a callback function can look like?

Thanks a lot!
lrrm