Plain agreements among competitors to divide sales territories or assign customers are almost always illegal. These arrangements are essentially agreements not to compete: "I won't sell in your market if you don't sell in mine."
Source: https://www.ftc.gov/tips-advice/competition-guidance/guide-a...

...anticompetitive market division schemes occur when competitors divide up customers or types of customers, determining who can sell to which customers.
Source: https://www.classlawgroup.com/antitrust/unlawful-practices/m...