What is Sandbox?
Sandbox, often abbreviated by sb or SB, is playing PSO with items from one of the various item makers that are maxed. They still follow the limitations (unless otherwise specified like in the 2014 meta) of the game such as 50 hit on enemy weapons or 100% in 2 elemental attributes or 1 on weapons with kill counters. These items are achievable in normal play, but they require 1000s of hours and a lot of luck to get.
Why play Sandbox?
The entire purpose of time attack is to optimize a given quest to its highest potential. If you're playing normal mode your items simply will almost never be optimal for the current run. You can still do your best of course and plan around what your items will allow you to do. In sandbox you may plan around perfect weapons and the big perk here is hit. The more hit you have the higher chance your weapons have of passing ATA checks. This reduces RNG by making attacks/combos more consistent. Players are all equal in their gear and must display their ability to plan and execute a quest rather.