difference between `wildignore=` and `wildignore+=` in vim

Question

I have seen many vimrc's which have wildignore+= and i don't know what it refers
compared to wildignore=

others like: clipboard= and clipboard+= some configs have also like clipboard-=

Can anyone tell me the difference?

Answer 1

:h :set+=

:se[t] {option}+={value}                *:set+=*
            Add the {value} to a number option, or append the
            {value} to a string option.  When the option is a
            comma separated list, a comma is added, unless the
            value was empty.
            If the option is a list of flags, superfluous flags
            are removed.  When adding a flag that was already
            present the option value doesn't change.
            Also see |:set-args| above.

If you have default empty wildignore then:

1. :set wildignore=*.o,*.obj will set it to the *.o,*.obj value
2. :set wildignore+=*.exe will set it to the *.o,*.obj,*.exe value