A EOI, or Electronic Object Identifier, is a string of numbers, letter and symbols used to permanently identify an article or document and link to it on the web. A EOI will help authors to easily locate a article from your citation. Think of it like a Unique Identifier for the article which is unique across the globe, and permanent. The meta-data associated with that EOI can be changed, but EOI will remain same.
A EOI can be assigned to journal publication, conference publication, thesis, websites/web portals or any online/offline published content.