What is %1$s %2$d used for https…

What is “%1$s %2$d” used for ?

https://core.trac.wordpress.org/browser/branches/4.1/src/wp-includes/general-template.php#L1409
https://core.trac.wordpress.org/browser/branches/4.1/src/wp-includes/media.php#L2853

Any examples ?

#help