The underlined phrases in each of the following English sentences are examples of vocatives: Sir, your table is ready.
Syntactically, vocatives are noun phrases which are isolated from the structure of their containing sentence, not being a dependent of the verb.
[1] Traditionally, the function of vocatives has been divided into two main categories: calls and addresses.
[2] A call serves to catch the attention of the person being addressed, or to pick them out from a larger pool of potential addressees, as in the following examples: An address merely serves to reiterate, clarify, or emphasize the connection between the speaker and the addressee, as in: The particular choice of vocative may indicate the relative social status or familiarity of the speakers.
For example, sir and madam are considered polite terms to use when addressing strangers or, in some cases, those of higher social standing.