The IRS is a federal agency governed by federal laws. The laws or decrees of a lower level court can not over ride federal laws. Therefore, the IRS does not recognize your divorce degree of custody agreement as authoritive on your federal tax return. Only a federal judge can over ride federal law. Since federal judges do not deal with divorce or custody issues, that will never happen. So you don't need to explain anything to the IRS so long as your follow the federal legal requirements as dictated in IRS Publication 504 at https://www.irs.gov/pub/irs-pdf/p504.pdf. Take special note that child support *does* *not* *count* as support for the child in determining which parent provided more than half of the child's support.
TO see an abbreviated version of the requirements you need to meet to claim the child as your dependent, see the table at https://apps.irs.gov/app/vita/content/globalmedia/table_3_children_of_divorced_separated_4012.pdf
