In Star Wars books, Darth Vader is frequently described as a "Sith Lord". So is Palpatine. So is Dooku, and Maul, and Plagueis.
But most of the time, in the period in which the movies (and most of the books) take place, there are only two Sith in existence at any given moment.
This would appear to make the honorific title "Sith Lord" mostly meaningless, because what good is it to be a lord if there is no one under you? Vader is effectively a lord of nothing and no one. Palpatine is the Emperor of the galaxy, but as a Sith Lord, he has only one subordinate (whether that subordinate is Vader, Maul, or Tyrannus) at a time (except for the period when Maul and Tyrannus both served him).
This being the case, why is Vader (or any other Sith who is subordinate to a more powerful Sith) a "Sith Lord" rather than just a "Sith"?
What, if anything, is the difference between a "Sith" and a "Sith Lord" during the period in which the movies take place, in light of the Rule of Two?
Note: For the purposes of this question, we will treat the honorific "Lord" as distinct from "Sith Lord", since everyone called Vader "Lord", but only he, Palpatine, and the Jedi knew he was a Sith. Thus, when an Imperial calls Vader "Lord", they aren't referring to his status as a Sith Lord.