How can I filter which entities are available in the admin form for an Entity Reference Field? I've got a Field which is called "Primary Article" which is an Entity Reference Field. I've configured this field setting the "Reference Type" to be "article", so that only articles show up in the Form when editing this field. 
I would want to add two customizations to the current autocomplete field:
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1) only return some articles based on some logic ( like I only want to return articles which are not selected as primary articles for another taxonomy )
2) change the text of the autocomplete field, so in this case, instead of "Primary Article" I would want to return "Primary article - XYZ" where XYZ is a custom field
1.) You can create the limited list of Articles with Views, then use that Views in the Entity reference Field settings as the source. 

2.) This should have probably gone into a separate question. Try researching into modules related to autocomplete, like: Search Autocomplete, Autocomplete Search Suggestions.