Here are some useful tables used by Tax Reporter (tcode PU19).
T51T0 – TAXTY and LGART
T5UTO – link between Tax Type (TAXTY) and wage types (LGART)
T5UTP – link between Tax form group (TAXTP) and Tax Type (TAXTY)
T5UTI – link between Tax authority (TAXAU) and Tax type (TAXTY)
T5UT3 – Tax form field (TXITM)