What does selection of the "Custom" content in the set-up menu do (or even the "Excerpt" option)?

My issue is that my source posts have title, description and an image - but then also some social media share & navigation buttons appearing below. I want to import (to the forum) only title, description & image excluding the buttons.

Currently everything on the post gets cross posted and I have to manually delete the rest by editing the forum post. Is there away I can automate this?