Special Kinds of Methods
Methods associated with a type rather than an instance of a type must be marked with the static declaration modifier for enumerations and structures, or with either the static or class declaration modifier for classes. A class type method marked with the class declaration modifier can be overridden by a subclass implementation; a class type method marked with static can’t be overridden.
Variable Declaration
A variable declaration introduces a variable named value into your program and is declared using the var keyword.
Variable declarations have several forms that declare different kinds of named, mutable values, including stored and computed variables and properties, stored variable and property observers, and static variable properties.
Type Variable Properties
To declare a type variable property, mark the declaration with the static declaration modifier. Classes may mark type computed properties with the class declaration modifier instead to allow subclasses to override the superclass’s implementation. Type properties are discussed in Type Properties.
In a class declaration, the static keyword has the same effect as marking the declaration with both the class and final declaration modifiers.
Protocol Method Declaration
To declare a class or static method requirement in a protocol declaration, mark the method declaration with the static declaration modifier. Classes that implement this method declare the method with the class modifier. Structures that implement it must declare the method with the static declaration modifier instead. If you’re implementing the method in an extension, use the class modifier if you’re extending a class and the static modifier if you’re extending a structure
https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#//apple_ref/doc/uid/TP40014097-CH34-ID483