The Legislative power of Union Legislature extends to all matters mentioned in the Union list and in the Concurrent list of the seventh schedule to the Constitution but does not extend to any of the matters mentioned in the State list of the seventh schedule. The Union Legislature has the residuary legislative powers. The division of legislative power between the Union and the States is an important feature of the Constitution. Legislative enactments of either the Union or the States are often challenged on the ground that the enactments are beyond the legislative competence of the Union or the States. The higher judiciary then decides on the legislative competence
Source:-Guide to Indian Laws- By V. Ramakrishnan