Composite and mixed supply rules determine tax liability: composites follow the principal supply, mixed supplies follow the highest-rate component. Tax liability for combined transactions: a composite supply comprising multiple supplies, one of which is the principal supply, is treated as a supply of that principal supply; a mixed supply comprising multiple supplies is treated as the supply that attracts the highest rate of tax, so tax liability is determined by the principal component for composites and by the highest-rate component for mixed supplies.