Dimensional Box

A box is an item you can use to store stuff in, as there is a limit to how many items you can carry at a time. There are a few physical boxes before Alphys upgrades your phone. After your phone is upgraded, two dimensional boxes will appear in your phone list and you can use them to move around items. If the player is doing a Genocide Route then Alphys does not show up to upgrade their phone, but 3 more boxes are placed to compensate for this.

The first time that you open the box in Snowdin, it will contain a free Tough Glove, which you can take free of charge.

Box Locations
Snowdin Forest Snowdin Waterfall MTT Resort (Genocide Route) New Home (Genocide Route)
 * Located on the road just after you meet Sans and Papyrus for the first time.
 * Located in front of the Snowed Inn and Shop.
 * Located outside of Gerson's shop
 * Located to the right just after entering.
 * Located right outside the house, in front of the right window.
 * Located in Last Corridor, beside the save point.

Trivia

 * If you try to use the dimensional boxes in The Piano with a Great Treasure room the message "The box is aclog with the hair of a dog" will be displayed. You won't be able to use your boxes until you leave the room.