To Close a Mesh Element to a Volume

Select the Close Mesh Voids tool.
Turn off Perimeter Less Than.
Select the mesh element.
(Optional) Do one of the following:

Select additional elements by pressing <Ctrl+ click>.

or
Select multiple mesh elements with a selection set.
(Optional) Press <Ctrl+ click> to change a yellow highlighted void to blue or to change a blue highlighted void to yellow.
Click to close the mesh element to a volume.

Original mesh element (left) and mesh element closed to a volume (right)