Best efforts sits at the top of the efforts hierarchy in contract law. It asks for more than “reasonable efforts,” which calls for sensible steps, and more than “commercially reasonable efforts,” which measures conduct against industry norms. A party bound by a best efforts clause is expected to exhaust available avenues, not just pursue the obvious ones.

The clause acknowledges that success depends on variables nobody fully controls: market conditions, regulatory approvals, third-party cooperation. So instead of promising a result, the party promises effort of a particular quality. Courts then evaluate that effort after the fact, which is where things get complicated.

From https://fedlaws.org/best-efforts-standard-in-contracts-ucc-r...

"Best effort" is below rigorous QoS specifications though. Saying "99.999% of calls must complete" with a rigorous definition of completion is different than saying "best effort".