Is the title "Dark Lord of the Sith" reserved for the Sith Master?
For the Sith Order under the Rule of Two (i.e. from <PERSON> to <PERSON>), does the Sith Apprentice hold the title "Dark Lord of the Sith"? Or is the title reserved for only the Sith Master?
In the (Legends) Bane trilogy, it appears to be reserved for the Sith Master. For example, from Dynasty of Evil:
That was why <PERSON> had chosen <PERSON> as his apprentice: she had the potential to one day surpass even his own abilities. On that day she would usurp him as the Dark Lord of the Sith and choose an apprentice of her own.
Later, Darth <PERSON> affirms that the Apprentice becomes the Dark Lord upon killing the Master:
The Master will train his apprentice in the ways of the Sith, until one day she must challenge him. If she proves unworthy, the Master will destroy her and choose a new apprentice. If she proves the stronger, the Master will fall and she will become the new Dark Lord of the Sith, and choose an apprentice of her own.
However, <PERSON> -- who never became the Sith Master -- appears to hold the title. He is referred to as the "Dark Lord" in the movie scripts, as well as in the canon novel Tarkin (albeit always from the perspective of Moff <PERSON>, who is not a Sith). Probably on this basis, Wookieepedia's canon article on the title says that the title was "shared" by the Sith Lords under the Rule of Two.
There appears to be a contradiction, in that the <PERSON> trilogy reserves the title for the Master but the canon sources do not. 605
How is this explained? Did the Sith Order start giving the title to the <PERSON> sometime after <PERSON> and, if so, when? Is the <PERSON> trilogy simply wrong to reserve the title for the Master (it's Legends, so canon supersedes it)? Or did <PERSON> not really hold the title?
The last option (that <PERSON> didn't really hold the title, but was publicly assumed to hold it) seems possible for several reasons:
1. <PERSON> is never called <PERSON> by <PERSON>, and never calls himself <PERSON>. He is only referred to as the Dark Lord from an out-of-universe perspective (e.g. in the scripts) or by non-Sith characters (e.g. <PERSON>) who wouldn't know his true title(s). Within the Sith Order, it's possible <PERSON> doesn't really hold the title.
2. <PERSON> was not known to be a Sith by the general public, so from the public point of view <PERSON> was the only Sith and therefore the Dark Lord.
3. Out-of-universe, there may be a bit of a retcon here: <PERSON> didn't initially intend <PERSON> to be a Sith and started calling <PERSON> the Dark Lord in the Episode V script; when the Emperor's character was changed to be a Sith, he was stuck with <PERSON>'s title. 605