Luminosity Class tells you which region of the H-R Diagram the star belongs in. Together, give a unique location on an H-R Diagram. Build up an H-R Diagram for nearby stars with parallax distances. Practical limit few 100,000 pc. Works best for star clusters.
Luminosity classes have some spread.
H-R diagram location depends on composition . Faint spectra give poor classifications. Stars Stars whose brightness varies with a characteristic, periodic pattern. Period of their brightness variations. Period-Luminosity Relation exists for certain classes of periodic variable stars.