This setting determines the maximum amount you want to invest in an individual borrower since they can apply for more than one loan with Bondora. It shouldn’t be confused with the possibility of setting the maximum bid size (click the link to learn more about maximum bid size).
If you set the "Max investment per borrower" higher than the "Bid size", Portfolio Pro may bid on the same loan multiple times. Your investment in one loan is then divided into several parts. In other words, auctions receive more than one offer from you. This approach can be beneficial. For example, it’s possible to sell single parts on the Secondary Market later, rather than the whole loan. On the other hand, you could argue this gives you less diversification overall.