Hindu marriage act 1950

Under section 5 of Hindu marriage act 1950 Whose copy is available on the website of highcourt The point 4th; the parties are not within the degrees of prohibited relationship unless the custom or usage governing each of them permits. The point 5th; the parties are not sapindas of each other, unless the custom or usage governing each of them permits of a marriage between the two; What is the meaning of "the custom or usage governing each of them permits of a marriage between the two"?? What "customs" are written and registered under the law?? And what is the "usage governing each of them" means??