First, NS_ENUM uses a new feature of the C language where you can specify the underlying type for an enum. In this case, the underlying type for the enum is NSInteger (in plain C it would be whatever the compiler decides, char, short, or even a 24 bit integer if the compiler feels like it).

Second, the compiler specifically recognises the NS_ENUM macro, so it knows that you have an enum with values that shouldn’t be combined like flags, the debugger knows what’s going on, and the enum can be translated to Swift automatically.

NS_ENUM allows you to define a type. This means that the compiler can check if you’re assigning the enum to a different variable like so:

//OK in both cases
NSInteger integer = SizeWidth;
//OK only with typedef
BOOL value = SizeHeight;

NS_ENUM also provides checks in switch statements that you’ve covered all possible values:

//Will generate warning if using `NS_ENUM`
switch(sizeVariable) {
    case SizeWidth:
        //Do something
}