NSString literals and strings-files use different escaping rules.
NSString literals use the same escape sequences as “normal” C-strings, in particular
the “universal character names” defined in the C99 standard:
\unnnn - the character whose four-digit short identifier is nnnn
\Unnnnnnnn - the character whose eight-digit short identifier is nnnnnnnn
Example:
NSString *string = @"Espa\u00F1ol - \U0001F600"; // Español - 😀
Strings-files, on the other hand, use \Unnnn to denote a UTF-16 character,
and “UTF-16 surrogate pairs” for characters > U+FFFF:
"spanish-key" = "Espa\U00f1ol - \Ud83d\Ude00";
(This is the escaping used in “old style property lists”, which you can see when printing
the description of an `NSDictionary.)
This (hopefully) answers your question
When to use “\Uxxxx” and “\uxxxx”?
But: As also noted by @gnasher729 in his answer, there is no need to use Unicode
escape sequences at all. You can simply insert the Unicode characters itself,
both in NSString literals and in strings-files:
NSString *string = @"Español - 😀";
"spanish-key" = "Español - 😀";