The string format, when added to string.xml resource, has a differente syntax, for exemple:

Hello, %s! You have %d new messages.

Becomes:

Hello, %1$s! You have %2$d new messages.

The syntax is: *%<position>$<type>*. where <position> starts at 1 and type is the Java String format like d, s, f.

Source: String Resources