NominalScale

NominalScale[{cat1,cat2,…,catn}] represents a set of unordered categories cati. 

NominalScale[<|cat1->lab1,…,catn->labn|>] also associates the category cati with the labels labi.

NominalScale[…,<|cati->labi,…|>] uses labi to represent cati in plots.

NominalScale[Automatic] automatically determines the categories.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.