Maybe it's in the C99 standard. I don't have it, so I can't really tell.
The optional prefixes to type, h, l, and L, specify the “size” of argument (long or short, single-byte character or wide character, depending upon the type specifier that they modify). These type-specifier prefixes are used with type characters in printf functions or wprintf functions to specify interpretation of arguments, as shown in the following table. These prefixes are Microsoft extensions and are not ANSI-compatible.