This format is named STDOUT_TOP, and will be used initially at the first invocation of the STDOUT format, and again every time 60 lines of output to STDOUT have been generated. These column headings line up with the columns from the STDOUT format, so everything comes out tidy.
The first line of this format shows some constant text (Page) along with a three-character field definition. The following line is a field value line, which in this case has one expression. This expression is the $% variable, which holds the number of pages printed - a very useful value in top-of-page formats.