The Materials listed in that dropdown are defined in the Scriptable Object “Materials” at “Assets/TheraBytes/Resources” (Or via the Menu "Tools -> Better UI -> Settings -> Select Material Definitions").
The Better UI Materials are reserved for materials without any properties as well as for materials which use the float2 TEXCOORD1 and / or the w-value of the float4 TANGENT semantics.
By using these Vertex-variables for the properties of the shader, it is possible to share one material but having different properties on each Image. This allows not only a faster workflow, it also increases performance if you have many Images using one Material (instead of using cloned materials).
The Materials List is not for materials with normal properties.