What is a restriction of competition and when is it a restriction by object?

“Our competitors are our friends, our customers are the enemy”, this could be the unofficial motto of a company in a cartel.  Even though this sentence is of course a simplistic definition, it shows abstractly some major aspects of a restriction of competition in terms of Art.101 TFEU: The competitors are friends, which implies that there must be a kind of consensus preventing them to take actions against each other’s interest. In other words, since they are friends, the competitors are not completely free in their behaviour. At the same time, the friendship consensus protects each of them from potential losses due to inefficiency, because a friend would not make use of the weakness of another friend. The customers on the other hand are the enemies, meaning that, at the end of the day, they will pay the price (in the truest sense of the word) for the inefficiency caused by non-existing competition.