Nouns of direct address are nouns used when speaking directly to someone or to a group. They are also known as vocatives, nouns of address, and nominatives of address. As with interjections, nouns of direct address have no grammatical purpose or relation to the rest of the sentence and are usually set apart by commas. Vocatives may also be modified like any other noun, in which case their descriptors are also set apart with them.
Allan, are you still going to the movie tonight?
The speaker of this sentence is speaking directly to Allan. However, notice that the subject is still you. The vocative Allan has no grammatical relationship with the rest of the sentence.
This, my faithful friend, is for you.
In this case, the vocative is friend because it is the noun being used to identify the listener. It is modified by my and faithful, which are also set apart by commas along with friend, but the entire phrase still has no relationship with the rest of the sentence.
You, sir, are a charlatan and a fraud!
Here the title sir is being used to directly address the listener. Again, it does not relate to the rest of the sentence, it only serves to identify the listener.
Hello everyone! What’s going on?
In this example the vocative everyone is also accompanied by the interjection hello. Notice that the exclamation point eliminates the need for a comma.