Most selections in-game are made with the soul moving from choice to choice, but there are certain instances where the soul is not used. Those being when making selections on the main menu like continue and reset, and name the fallen human. Those choices are made in yellow. There's one instance where your choice is selected with both the soul, and yellow text, and that is "erase" or "do not". What do you think about that?
This is one of those moments where the game’s format both works for and against ascribing a message to individual elements. Toby has designed Undertale to play around with the rules fairly often, but there are a few moments where gameplay and general design choices may clash, should this be consistent at all times. Take the naming screen, for example.
This is a very classic-style naming screen, featured in many-many games throughout the years. Having to select each individual letter is a throwback to the oldest naming conventions in gaming, where a D-pad and an ‘A’ button were the only options and is considering Toby’s influences, it has to be more of a stylistic, than functional choice.
However, notice the gap between the columns and lines. Considering Undertale naturally starts in windowed mode, adding a soul-selection there, rather than yellow text, would add a lot of clutter and in some case make it mildly difficult to tell which letter the player is about to select. So while it’s arguable, this may very well be a deliberate choice of convenience.
Narratively, it can be argued that at that point in the game, the player is merely about to give a name to the fallen human. In a sense, this is how a ‘gateway’ is created from player to the game, starting all the way with the initial description of menus and the assigned control layout. The soul and Frisk don’t come into play until after the naming is complete. It could be argued that since the main menu exists for the player, the individual’s determination is the only thing needed to advance. This line of thinking leads us to the following exchange as well.
Out of all the multiple choices in the game (besides the act of resetting), this is by far the most significant and with the most far-reaching implications. The world lives or dies depending on this choice, though at this point in time there is no way of knowing that both choices lead to the same outcome.
In some parts of the game (notably Flowey and Asgore’s fates), the UI changes somewhat to grant a bigger impact to the moment. While this is a change as well, it is clear that this is different. *FIGHT and *MERCY have no meaning in this conversation and yet the choices are just as significant. Once more, there may be a functional and a narrative aspect to this design choice. Functionally, it helps underline the significance of this moment and allows the player to grasp (or at least suspect) the seriousness of this decision. Narratively, it could be argued that due to the impact, this is a decision that can only be made not only by soul (ironic considering what can happen afterwards), but also every ounce of determination.
As they say, the hardest thing about setting a bomb off, is to actually push the button.