Generally, obligations under a contract are absolute, with a failure to adhere to one's obligations resulting in a breach of contract. Endeavours clauses are different – they simply refer to a party, or parties, endeavouring to achieve a specific objective or to fulfil a particular obligation. For example, "A shall use best/all reasonable/reasonable endeavours to ensure that Part A of the workscope is completed by 10 December 2020".
Issues arise when parties find that they are in dispute as to the extent to which endeavours obligations have been fulfilled. Generally, there are three "levels" found in endeavours clauses.
This is the most burdensome standard. It is probably because of this that it has most often been the subject of consideration by the courts. A widely accepted starting point is that the term "means what the words say; they do not mean second-best endeavours". Essentially, it is an obligation for the obligor to do everything in its power to achieve the objective set, even if such actions are to its own detriment.
All Reasonable Endeavours
This is often seen as a middle ground between best endeavours and reasonable endeavours. An obligor faced with an obligation to use "all reasonable endeavours" will be expected to explore all reasonable courses of action available to it. Where a court finds that there were reasonable steps which an obligor could have taken but failed to do so, or that the obligor has not explored all options, it will be reluctant to conclude that the obligor has used "all reasonable endeavours". Generally, the obligor will not be expected to sacrifice its own interests in respect of "all reasonable endeavours", unless the contract states otherwise.
In order to demonstrate that it has used reasonable endeavours, an obligor must show that it has taken a reasonable course of action to achieve the objective. If an obligor can demonstrate this, there is unlikely to be any further obligation upon it in respect of achieving the objective, even if other avenues are available. Generally, the obligor will not be expected to sacrifice its own interests in respect of "reasonable endeavours", unless the contract states otherwise.
As in any contract dispute, a contentious endeavours clause will be subject to interpretation by the courts, with the usual rules of interpretation being applied to the applicable facts. The ideal negotiated position will, of course, depend upon whether you are the obligor or the obligee.