The IAAF doesn't have a formal editorial style guide for the relative use of these terms, so in practice they're interchangeable as Ben Miller answered. I'd be interested to see if there are similar synonyms in French and Italian, as French is also an official language of the IAAF and their primary statisticians are English and Italian.
As terms of art, the two words are often used to distinguish when an athlete appears on e.g. an annual list multiple times: Bolt might have six performances in the ten best for a given year, but for other athletes down the list, only their best mark is listed.
In other contexts, e.g. a field event which is ongoing, an athlete might have three existing marks "on the board", but their performance will be the best of those marks when the competition is completed. But I wouldn't want to suggest either of these as rules which are followed rigorously.