How to set plugin required elements for automatic Advanced Content Filter?

The docs explaining plugins integration with ACF aren't ready yet, but they will be published here (I'm posting this link so in the future people can easily find it :).

First of all, please read the CKEDITOR.feature doc. It may help you understanding the concept that is used. Also, if you haven't seen it, check ACF guide.

Basically, you don't say that "plugin allows img tag" (at least not now). Plugin may include many features allowing many things (e.g. basicstyles adds 4 buttons). You rather say that "this button, when is in toolbar, allows some content". Therefore you can set property of the button's definition that tells that, or even better, if button just triggers a command, then in this command's definition.

So as you can see, a command and button have something in common - they represent the concept of the editor feature which in the code is an interface. That's exactly what CKEDITOR.feature is. Currently only buttons, richcombos and commands implement it, but in the future plugin may do that too.

Anyway, this magical property of a feature that tells the editor that this feature, when it is activated (e.g. button is added to toolbar), allows some content is CKEDITOR.feature#allowedContent. Add it to button's, or better, command's definition and voila!

Second important property is CKEDITOR.feature#requiredContent which tells editor what is the minimal HTML required by the feature to be activated. E.g. image plugin allows 'img[!src,alt,width,height], but to work it needs only 'img[!src]' (+ of course alt if it has to produce valid output).

Check our plugins code where you'll find how we integrated them with ACF. Hopefully, this week we'll update guides.

BTW. I forgot to mention that dialog integration is also possible. You can use requiredContent flag on dialog ui elements such as fields and tabs. They will be hidden if not allowed.