Orderless

Orderless is an attribute that can be assigned to a symbol f to indicate that the elements in expressions of the form f[e1, e2, …] should automatically be sorted into canonical order.

Examples

Define orderless function:

SetAttributes[f, Orderless];
f[c, a, b]
(* f[a, b, c] *)

Check attributes:

Attributes[Plus]
(* {Flat, Listable, NumericFunction, OneIdentity, Orderless, Protected} *)

Please visit the official Wolfram Language Reference for more details.