Optical properties of the surface

what is the meaning of the REFLECTIVITY and EFFICIENCY in the optical properties of the surface?
G4double reflectivity[num] = {0.3, 0.5};
G4double efficiency[num] = {0.8, 1.0};

G4MaterialPropertiesTable *myST2 = new G4MaterialPropertiesTable();

myST2->AddProperty(“REFLECTIVITY”, ephoton, reflectivity, num);
myST2->AddProperty(“EFFICIENCY”, ephoton, efficiency, num);

You may find the discussion of optical physics useful, if dense. Those properties are discussed around “Listing 68”.