@ItsA1WayStreet
I think where things are tricky is the use of the definition "best reasonable effort". Is this a legally defined, well used term in business contracts? If not, who is to say what is "reasonable"?
It's a very, very commonly used concept.
In English law we tend to use best endeavours/all reasonable endeavours/reasonable endeavours. There's a lot of case law about what these mean, and also parties can be really specific in the contract as to what it means to do this (as they have done here in the definition of BRE). They essentially all mean "I will try to do this but cannot promise anything beyond that" (as opposed to just saying "I will do this"). "Best endeavours" means that you'll try really really hard to do it, "reasonable endeavours" means that you'll try but you won't put your back out doing it.
Unfortunately, the AZ contract is governed by Belgium law, and goodness knows what Belgium law says about how this should be interpreted!