Summary
Sandbox is a Universe that allows the player who joins it do whatever he/she desires. It can be useful for testing weapons out before actually buying them, making it public and adding more challenges, or just want to simply chill in maps with music you haven't heard of yet.
More in depth info
Upon launching Roblox to open R2Da, you are greeted with a new button on the left side of Single Player. Once clicking/tapping the button, you are sent to the Sandbox Universe, where you are greeted like any other normal R2Da gameplay, only there is a new button named Controls, on the left side of the Intermission. When you open the store, you immediately have infinite cash and tickets, have unlock Everything in store, and every mount and event items/armors/weapons. Once you have finished equipping the weapon/armors/items and mount of your dreams, you can now choose the maps, along with any gamemode.
When clicking on the controls, you are given a guide on how to use vip commands, choosing the maps, gamemodes, and even change the music.
All give commands are infinite, so have fun spamming water balloons.
Reload stations are infinite too. If an event is going on, you can choose the quest or go straight to the boss battle. Jetpack fuels are infinite, along with timers and hammer structures.
When you join, the server is set in single player. In order to change it, you can either make it public to select few people (like friends) or just make it public to anyone.
Keep in mind, stats WILL NOT transfer to any other server other than Sandbox universes.
Trivia:
Sandbox has been mainly inspired by TABS (Totally Accurate Battle Simulator) in which is had a sandbox mode, with infinite money and can change the map.