Все зависит от контекста и юрисдикции. В европейских судах established business relationship означает: The Paris Court of Appeal recently recalled that for a business relationship to be established “there must exist
a sufficiently prolonged, regular, significant, and stable flow of business that augurs the continuance of the
business relationship” (CA Paris, 18.01.2017, 14/08437).
The court’s test appears to include temporal (“prolonged”), quantitative (“significant”) and qualitative
(“regular”, “stable”) factors cumulatively.

Вот определение Nixon Peabody: Established Business Relationship (EBR): A prior or existing relationship between a business and an individual based on the individual�s purchase or transaction with the business within the 18 months preceding the call from the business or based on the individual�s application for products or services from the business within the 3 months preceding the call. 47 C.F.R. � 64.1200(f)(5). http://web20.nixonpeabody.com/tcpa/pages/What-Is-An-Establis...

Российскую специфику таких "отношений" пояснять излишне))

Думаю, здесь может идти речь об официальных деловых (коммерческих и как-то оформленных) отношениях и прочих тесных деловых связях. Смотрите по контексту и характеру отношений между сторонами. Надеюсь, что-то прояснится)