'Filler' flowers are little buds or blossoms that are easy to carve and join together to fill spaces between the large flowers. In the swag proper, we'll be trying to lose as much empty space between the flowers as possible, which means less work hollowing out and undercutting.
I'm not quite sure what this 'flower' is meant to be, but it's a frequent visitor to the climes of Baroque flower carving, with lots of possible variations.