At the beginning of the game, you have the option to choose between the Standard Sphere Grid and the Expert Sphere Grid. While the former is more familiar to fans who may have played Final Fantasy X in the past, the Expert Sphere Grid might be new to those who never played the International version on PS2. Basically, all characters will begin in the center of the Expert Sphere Grid. This allows you to make different choices in a character’s build, so you’re not stuck on a pre-determined path.
For example, Lulu is pretty much the Black Mage of the group if you pick the Standard Sphere Grid. However, if you pick the Expert Sphere Grid, you can make her into a White Mage instead. Characters are not defined by the rules of the normal Sphere Grid, so they can deviate a lot from their normal roles. There are still locks throughout the Sphere Grid, like Use and Steal being blocked by a Lv. 1 lock. It takes a lot more planning to use the Expert Sphere Grid, so it’s not recommended to beginners. Note that the Expert Sphere has around 50 fewer nodes over the Standard Sphere Grid, so this might affect which stats you prioritize in the endgame, as far as maxing them out goes.