Hi Antje, thank you for finding this entry. It does seem to match the context of my query. I am just a bit worried because the entry on the economypoint.org is written in a peculiar form of English, so I am not sure whether to trust it. I searched for "profit border" on the Internet but have so far not come up with a definition that matches my original context. I have found a great German graph which exactly points out where on the curve the "Gewinngrennze" or "Nutzengrenze" as they also call is to be found:
http://www.bilbuch.de/Stichwortverzeichnis/Nutzengrenze.htmlThere are numerous German exam questions etc. on the internet containing the term "Gewinngrenze", so there must be an equally well known English term around. But I was not even able to find this kind of curved graph in English. Very peculiar, indeed.