The buttons are below one witch is marked “# Mat: #”
with “new” you create a new index, for the object, for assigning a material to a selection of vertices.
As many indices you have, as many material you can assign to your mesh.
with “delete” (you gess)
under, you have “select” and “deselect” for selecting (or deselecting the vertices that depends on the active index.
finally, you have “assign” for assigning the vertices selected the active index.
My english is so far to be perfect, so I hope being helpful!! :-?