For me it's not clearly described what proceedings-published posters are. Here is a CFP...
I've not heard the term before, but the description seems clear enough from their website:
Participants submitting proceedings-published posters/demos are required to submit a short paper of 2 pages describing the poster/demo content ... If accepted, this short paper will be published in the proceedings of the conference.
In other words, they will publish an associated short paper in the proceedings as well as displaying the poster. This is distinguished from the other option for a paper, which will be displayed, but not have anything published in the proceedings.