[Help-bash] When will COMP_TYPE be ! or @?

[Clark Wang]

According to the manual:

COMP_TYPE
    Set  to an integer value corresponding to the type of completion
    attempted that caused a completion function to be  called:  TAB,
    for  normal completion, ?, for listing completions after succes-
    sive tabs, !, for listing alternatives on partial  word  comple-
    tion,  @,  to list completions if the word is not unmodified, or
    %, for menu completion.  This  variable  is  available  only  in
    shell  functions  and  external commands invoked by the program-
    mable completion facilities (see Programmable Completion below).

I can get TAB, ? and % but I don't know how I can get ! and @.

-clark