[
Means FIRST, [default] or Start Class (in Regexp)
Left bracket ([) is short for the FIRST line in the file, by default. This abbreviation can be changed with Set Zip.
/list [ {list first line}
In questions, Qedit shows the default answer in square brackets:
Purge existing file [No]?
In the documentation, [ and ] indicate optional fields in commands (e.g., List [rangelist ]).
Left square bracket ([) in regular expressions indicates the start of a character class.
/List "x[abc]z" {character class starts with "a"}