What's the difference between %ul and %lu C format specifiers?

Question

In an example of \"C Primer Plus\", the author has used %ul format specifier (in both scanf and printf) for unsigned long. When I try to generalize

Answer 1

%lu is correct, while %ul is incorrect.

A printf format specifier follows the form %[flags][width][.precision][length]specifier.

u is a specifier meaning "unsigned decimal integer".

l is a length modifier meaning "long".

The length modifier should go before the conversion specifier, which means %lu is correct.