For programs that are concerned with producing meaningful or clearer output. A typical example would be the transformation of a decimal into a particular currency format with the correct number of decimal places. (Note that code layout is always in scope for answers, so this tag is not used to indicate perceived issues with that).
There are two aspects to formatting:
How the output is presented, such as in columns or as a chart.
Formatting data items
How individual items are converted from internal representation to human-friendly output.