What is a PhD forum? Is it similar to short paper submission? I am a Ph.D. student in Electrical Engineering and I have been looking at submitting a paper to one particular conference I wanted to go to. In the end, I have missed the deadline for full paper submission. However, I still can have the option of doing a Ph.D. Forum submission. I been to several conferences already, but I am not really familiar with a "Ph.D. Forum".

What is a Ph.D. forum exactly? Is like a short/poster paper? (2-4 pages?) Is work still publishable?

My advisor tells me that it could be good idea for me to do it anyways, if I want.
The nature may vary from conference to conference, but in my experience a Ph.D. forum is typically an event intended to help graduate students learn how to present and how to shape their papers to fit the expectations of the field.  It is often a place for students to start getting advice and feedback on their work from the larger community, and for the larger community (who may be interested in hiring the student in the future) to start getting to know them.

As such, it is typically very lightly reviewed and only sort-of counts as publication.  As such, it is not worth very much on your resume (the value is in the networking and feedback), but it will not usually prevent you from later publishing a full version of the same work elsewhere.