3-4 verse 4-3 verse nickel verse dime verse short yardage.
Each formation comes with its own terminology, references, and personnel responsibilities, yet there is still considerable overlap.
It really doesn't really matter how the positions are labeled; it is more about who is responsible for what.
Regardless of formation, generally speaking: People still have to provide deep hashmark and seam help. People still have to cover "hook, curl, and flat" in the short zone. People still have to pressure the quarterback. People still have to set the edge and turn runners back into pursuit. People still have responsibility for specific gap control in the run game. People still have to disguise their intentions for the QB pre snap read.
It is a matter of first defining the alignment, and then associating the proper responsibility to any given position, with the alignment responsibility defining the terminology of how a given position is referenced (the name given to that position).
In short, there is no answer to your question without first declaring the formation. That is the starting point. Further, the advent of the "zone blitz" has basically wiped out any given absolutes with respect to defining pass coverage (by position). Even a nose tackle can drop into a short zone. Hence, "LB's cover, but DE's don't cover" isn't even a helpful definition anymore (unless you are talking in general about what typically happens). Everything nowadays is an attempt to deceive the quarterback's eyes. The more that teams implement "positionless" players, the more convoluted the terminology becomes. The basic responsibilities listed above, however, never go away.
On Edit: I failed in a big way to address field position in my response. There is probably no bigger factor that influences defensive formation choice than field position. Think of it like this: The more field that lies ahead of an offense, or in other words, the more open field that an offense has to work with, then the more options that are available to that offense (one must also take into account the fear of turning the ball over when close to your own goal line). Regardless, as the field "shrinks" so do the offensive options. For example, when in the red zone, the defense can eliminate the responsibility of deep pass coverage. This frees up the safeties for other roles like double coverage, run blitzes, or more manageable zone responsibility. Gap control takes on more importance for the linebackers as the safeties assume responsibility for the LB's short zones. There is far less danger in playing man coverage, because the available receiver routes have shrunk to just a few. When man coverage becomes less dangerous, then blitzing unexpected players becomes more advantageous.
Field position management leads to the "bend but don't break" philosophy. If you don't give up the "big play" touchdown, then you actually have decent odds of forcing a field goal if the opponent reaches your red zone because opposing offensive options diminish in scope as the field shrinks. Further, sustaining multi-play, incremental offensive drives can be easily stalled with unforced offensive errors such as penalties or a multitude of other mental mistakes. All of this plays a huge role in how a defense will choose its formation.
