If checked, then the character frequency table will only show the letters in the file, lowercase and capitals combined.
If the box is not checked, the character frequency table will display all ASCII characters contained in the file, whether or not they are 'printable'. For example, spaces are identified by the ASCII number and by the message 'space'. Printable characters are displayed as the character iteself, whether or not the font in use supports them (if not, they will be displayed as rectangles).
