When someone wants to start a new article which can be summed up as "the same as Article X, only with <specification>"—or may alternately be phrased as "a combination of article X and article Y".
This is generally not a very good idea, since it can easily clutter the wiki with nigh-indistinguishable trope variations. On the other hand, if the added qualification is clear enough that the result serves a distinct narrative function from the original—then it becomes a Sub-Trope.
Whether a given proposal qualifies as a Sub-Trope or The Same But More Specific can be highly subjective; Tropes Are Flexible, after all, but we don't need a new page for every last variation possible. Please consult your fellow Tropers first to get opinions.
If the added distinction is so narrow that the result fails to actually establish a pattern, the trope is Too Rare to Trope.