A format field is a special character, or several characters, preceded with %. Format fields are used in strings used with functions str.format, out, outp, mes and end. Such string is followed by arguments that will replace the format fields.
%[flags][width][.precision][h|l|I64|L]type
type - character that determines whether the associated argument is interpreted as a character, a string, or a number.
flags - character or characters that control justification of output and printing of signs, blanks, decimal points, and octal and hexadecimal prefixes.
width - number that specifies the minimum number of characters in output.
precision - number that specifies the maximum number of characters printed for all or part of the output field, or the minimum number of digits printed for integer values.
h | l | I64 | L - optional prefixes to type, that specify the size of argument. For arguments of type long, use I64. Note: here I is uppercase i, not lowercase L.