When is other state law relevant

I was listening to the argument before the Supreme Court of Virginia in Everett v. Tawes, which seemed pretty high-powered to me, and part of the discussion was about the language in the Virginia Code that makes each installment of child support a judgment by operation of law, and part of the discussion was about the relevance of the law of other states on retroactive modification of support. Congress has required the states to make their child support laws include certain provisions as a condition of their participation in the federal welfare program. And so, the child support laws of the 50 states are quite similar with respect to those certain provisions, and the law of spousal support incorporates the same provisions, sometimes in some places. So, it might be that the law of many states includes a provision that each installment of support becomes a "judgment by operation of law, with the full force, effect, and attributes of a judgment ... including the ability to be enforced," to comply with 42 U.S.C. 666(a)(9). The state laws might be similar enough to provide some useful precedents, like the decisions involving the application of Uniform Laws in other states.