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So the one can stand alone the other cannot.
съесть + type of food
поесть + nothing

At the moment I am satisfied with these. I don't think I will be able to memorize more uses.
So
1. Я поел один бутерброд - I haven't finished it. I just bit some pieces of the sandiwch
2. Я...

What is the difference?
100% identical for me.