Definitions for "Fill character"
Fill characters and nonprinting characters are transmitted to allow the receiver enough time to process data already sent. The ASCII characters NUL and DEL are most commonly used as fill characters.
A black box or vertical bar in workforms that represents content that must have valid data.
A marker, found in certain positions of fixed and variable fields, that indicates where a valid character must be entered before the system will accept that field. Fill characters appear on workforms. On most screens, it appears as a symbol _ .