Question

What the difference between external variable of some sort and static variable?

//ClassA.m
NSString *var1;
static NSString *var2;

@implementation ClassA

...

Answer 1

extern

An extern constant/variable is one which one definition may be accessed (or referenced) across multiple object files. It’s a global, exported C symbol. Use this if you want access to the constant/global variable from multiple translation units (or compiled files) or if you want to use it in multiple binaries (e.g. you want to use it from your app, and the definition is in a dynamic library). You will typically declare this in the header for others to use.

static

The static emits a copy for each translation. Each file that is compiled which also sees (e.g. is #included) the static will emit a copy of that static. You should avoid this. This will result in a bloated binary that has very confusing execution. You should favor the static if the value is local to a file, and should be private to that file.

For these reasons, you should favor static in your .c, .m, .cpp, and .mm files, and extern in your headers.

Finally, that NSString pointer should be const by default:

// declaration - file.h
extern NSString * const var1;

// definition - file.m
NSString * const var1 = @"var1";

or

static NSString * const var2 = @"var2";

If you still want more, here’s a related answer I wrote, and here’s another.