Undertale comments on other games. This is apparent as soon as you hear its tagline: "The friendly RPG where nobody has to die." This calls to mind RPG conventions, and it works best if you get what the term "RPG" implies. You will think of story-heavy games where killing monsters is required to progress. Once you get that, the promise of nobody dying makes sense. And just in case you missed the point, Sans explains that EXP and LV--typical RPG terms--are EXecution Points and Level of Violence.
Undertale also encourages discourse of itself. Its limited save files prevent one player from easily experimenting to find every possible outcome for situations. While not unique to Undertale, it works nicely within the game as a whole.
How else does Undertale take part in or encourage discourse?