An appositive is a noun or a noun phrase that sits next to another noun to rename it or to describe it in another way. For example:
1. Mrs. Brown, the retired teacher, is coming to the school.
2. Tom the cat loves Jerry the mouse.
3. The car, the black Mercedes, is very classy.
In the above sentences, "the retired teacher", "the cat" and "the mouse", and "the black Mercedes" are the appositives to "Mrs Brown", "Tom" and "Jerry", and "the car".
The confusion now, majorly, comes with commas — when they should be used and when they should be omitted. As you can see, I have used a comma before and after some appositives but not others.
One general rule of thumb, not only for appositives, is that parentheses/nonessential information/added infomation should be set off by commas, one before and another after. Another rule is that any word or phrase that interrupts a sentence should be set off by commas, which I think parentheses naturally do.
Rule: When an appositive is essential to the meaning of the noun it belongs to, don't use commas. When the noun preceding the appositive provides sufficient identification on its own, use commas around the appositive.
https://www.grammarbook.com/blog/commas ... alifornia.
If the appositive is essential to the meaning of the phrase then we don't have commas
https://prowritingaid.com/grammar/10080 ... appositive
Where the subjectivity and confusion comes, however, is what exactly is essential and what is not. For example, in the first sentence, there could be other Mrs. Brown's, and, therefore, "the retired teacher" could be mandatory to set her apart, requiring no commas. Also, it could just be another added information hence necessitating the commas. This same argument can apply to all the remaining examples.
But you'll notice that "the black Mercedes" clearly interrupts the sentence, thereby making the commas necessary, but I think the same argument above can be applied here: what if the information was necessary, even though it does interfere with the sentence?
All I'm trying to point out is that there is a whole lot of gray areas with appositives — especially, I think, what is essential or nonessential, generally. What do you think about this?