The Instance Material operator can be used to assign a custom material to the render instances of applicable particles.
The materials applied by this operator only affect render instances (ex: VRay instances, assigned within a Mesh operator). This operator cannot be used to apply materials in general. In order to assign materials to particles in general, apply the material to the tyFlow object itself. If you need multiple materials, assign a Multi/Sub material to the tyFlow object itself, and use Material ID operators to differentiate which particles should receive which material.
No material: clears any material overrides applied to particles.
Scene Material: the scene material to apply to render instances of applicable particles.
.mat file: the material file whose first material will be applied to render instances of applicable particles.