choosing between $0 and BASH_SOURCE

How does one choose between "$0" and "${BASH_SOURCE[0]}"

This description from GNU didn’t help me much.

    BASH_SOURCE

 An array variable whose members are the source filenames where the
 corresponding shell function names in the FUNCNAME array variable are
 defined. The shell function ${FUNCNAME[$i]} is defined in the file
 ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}