"Level to show in gallery" determines which level is shown as the top level
in the preview under Home tab | Multilevel List; the specified level will
also be applied to text when you click the preview. Note that it is
recommended to associate each numbering level with a paragraph style (which
makes it easy to apply numbering to text), so you don't really need the
"Level to show in gallery" option.
"ListNum field list name" is the name that you want to use to display this
list in a LISTNUM field. Say that you type "mylist"; then you can use {
LISTNUM "mylist" \1 } to insert a level 1 item from the list anywhere in the
text.