When deciding what should be set off by commas in a sentence, do the words “essential” and “nonessential” mean grammatically nonessential/essential or nonessential/essential to the meaning of the sentence?
For instance, take this sentence: “He was the smartest and most ambitious man at his firm.”
Is it a matter of subjectivity as to whether “and most ambitious” should be set off in commas? Clearly, the sentence would work fine without it. In that sense, it is nonessential.
But the writer may decide that it is as essential to the meaning of the sentence as the information about the man being the smartest individual at his firm. Is it, therefore, the writer's choice as to whether or not to set it off with commas?
Or, since it is grammatically extraneous, should it be set off in commas?