All text between a ! character and the end of line is treated as a comment. Comments are most useful when entered into a script. Since there is no difference between reading from the terminal or reading from a script, comments can also be entered interactively.
There are two types of comments. If the first character in the line is ! then the entire line is ignored and the next line is read. If ! is in the input line, but is not the first character, the ! and all following characters are removed. Thus if the software requires that you enter a blank line (i.e., default) and you wish to include a comment, then you must include one or more spaces before the !.
Examples:
xsel > show ! This is a comment xsel > ! this line is completely ignored xsel > ! this causes the program to read a blank line