A character that is to be used to “fill” blank positions in a (text) form field or the client area of a window when it contains no form or display.
Any single character. If no character is specified, the space character is used.
The fill character is not data; it simply delineates the dimensions of a blank or partially-blank object on the screen.
In a field, the fill starts to the right of the last non-blank character (alphanumeric fields), or to the left of any number (numeric fields).