What "-a" operator means in "if" statement

Hi
I am trying to figure out what the following line does, I work in ksh88:

[[ -a /bb/bin/${big}.tsk ]] && LIST="$big $LIST"

Not sure what "-a" means in that case.
Thanks a lot for any advice -A

It just means "if the file exists". See the Conditional Expressions section of the "man" page for your Shell.