SAVE

In Undertale, SAVE is the in-universe name given to save files and the ability to overwrite them. There is one SAVE slot in the game, and it can be overwritten at several SAVE Points. In the dialogue, "SAVE" is noticeably written in caps, and it can be conjugated.

Load
A SAVE can be loaded via the game's menu. The game automatically loads the last SAVE when the protagonist's HP reaches 0. Meta knowledge plays a heavy role in this, where the protagonist and other characters remember events occurred before loading their SAVE; for instance, Toriel remarks that speaking with the protagonist is like speaking with an old friend, while Papyrus feels that they look familiar.

If the player tries to load the SAVE with corrupted data or in an inaccessible room (for inaccessible rooms, the case that "Dogcheck" was not turned off), the game will show the Annoying Dog in the middle of a black screen, rendering the game unplayable until it is fixed with a functional room or a fresh SAVE.

Reset
The Reset function is found in the game's menu. This function erases information stored in a SAVE, such as the protagonist's LV and EXP; resetting, however, does not erase certain variables such as the name. Similar to loading a SAVE, some characters in the game will act differently after a reset.

True Reset
A True Reset is a function available after completing the True Pacifist Route. It thoroughly erases all of the information stored in the SAVE file, with one exception.

If the Genocide Route is completed, the First Human destroys the world and its timelines. They then offer the player to recreate the world in exchange for their SOUL. Accepting this offer is similar to a True Reset; however, accepting this offer will also activate a permanent flag in the game's files, which affects the outcome of the True Pacifist Route.

Main Story
In the universe of Undertale, SAVING the game manipulates timelines. The use of this ability is exclusive to the one with the highest determination. The first character known to acquire this ability is Flowey, because of the determination he was injected with. His ability to SAVE is lost when the protagonist enters the Underground, as their determination supersedes his. Flowey is also noticeably the only character who retains full knowledge of the protagonist's actions before and after loading a SAVE, as opposed to other characters who only hint at it.

At the end of the Neutral Route, Flowey regains his ability to save after absorbing the six human SOULs. His use of the SAVE ability in this states is analogous to save-states, which he uses in battle to disorient the player. Another noticeable difference is the number of save slots; while the protagonist can only use one SAVE file, Photoshop Flowey uses four (SAVE files 1, 2, 3 and 6), although it can be inferred that he can use at least six.

No SAVEs
It is optional to save, and it is possible to reach all endings of the game without saving. Towards the end of the Neutral or Pacifist Routes, the game will acknowledge that the protagonist has not saved at all.

Neutral Route
Flowey will mock the protagonist for having made no SAVE files. He makes use of his ability to save and load as usual.

In the post-ending phone call, Sans may make a joke, suggesting that the protagonist should save. Flowey's monologue to the protagonist is unaffected.

After completing the game in the Neutral Route, the game will restart, and without any save file, the player will be prompted to begin a game in the menu.

True Pacifist Route
If the game has been completed at least once in the Neutral Route, then Alphys's sidequests are made available, making it possible to play through the game on the Pacifist Route. All of this can be done without saving the game.

During the final battle, the protagonist considers saving over Asriel's file to defeat him, but it is stated that since they never saved before, they lack the power to do that. The story continues as usual, but the game silently saves itself, preventing the timeline from being lost after the game is closed.