The various letters after type number (see below) refer to the dielectric material used.

Air or plastic foils can be used as dielectric material.

The line itself consists of a single conductor coated with dielectric material.

The structure is either hollow or filled with dielectric material.

Almost any dielectric material can act as a perfect mirror through total internal reflection.

In the general case, the dielectric material may be different above and below the central conductor.

Some dielectric materials are capable of acting both ways.

It is an important element of theories of dielectric materials.

So there is no qualitative difference between a metamaterial and a natural dielectric material.

This is approximately true for most dielectric materials, but not for some other types of material.