Forms of Address and Titles
Imperial culture is fairly formal, especially when it comes to the Sith. When addressing Sith an Imperial will usually say my Lord, even if the Sith is not a Lord. This also applies to Sith speaking to other Sith as well, especially if they are not known person. Another common way to address the Sith is to simply call them Sith especially if they are not a Lord or Darth. This can be confusing to non-Imperials, as they assume all Sith are lords. There is also the often used title of Dark Lord of the Sith, this is not an official rank, though at one time it probably had some significance now it simply denotes a particularly powerful Sith Lord or Darth. Technically this title should be reserved for the Emperor, but in recent times it has come to mean exceptional Sith, more so than the most powerful. All members of the Dark Council are essentially Dark Lords of the Sith, for example.
If they have a specific title or position they may simply be referred to by that such as the Emperor’s Wrath, these are rare exceptions. Imperial military tend to use their rank and name even in day to day life, especially if they are of high rank and position.
Slaves are rarely addressed by name, and common folk rarely have titles and those of nobility have varying forms of address associated with their social position. They are still beneath Sith, though they may have considerable pull and influence.