I learned about %1$@, %2$@ etc. today thanks to Wordcrafts

PlaceholderOrder.m

#import <Foundation/Foundation.h>

int main(int argc, char *argv[]) {
	NSAutoreleasePool *p = [[NSAutoreleasePool alloc] init];

	NSString *foo = @"foo";
	NSString *bar = @"bar";
	
	NSString *oldSkool = [NSString stringWithFormat:@"%@ %@", foo, bar];
	NSString *original = [NSString stringWithFormat:@"%1$@ %2$@", foo, bar];
	NSString *reversed = [NSString stringWithFormat:@"%2$@ %1$@", foo, bar];
	
	NSLog(@"oldSkool: %@\noriginal: %@\nreversed: %@", oldSkool, original, reversed);

	[p release];
}

// Output:
// 2014-09-26 14:48:56.731 PlaceholderOrder[38717:507] oldSkool: foo bar
// original: foo bar
// reversed: bar foo

// Source: https://www.wordcrafts.de/nice-to-know

Here's the link in a clickable version: https://www.wordcrafts.de/nice-to-know

Be sure to check out the pitfalls section at the bottom of that page.