If a divorce decree states you can ask for your ex-spouses Tax Documents in Good Faith, what does that include?

What are Good Faith reasons to ask for your ex’s most current years taxes. I have a predicament where the ex makes a hell of a lot more than I and I have my son full time plus and pay for everything. We have 50/50 joint and physical custody. I get 40 bucks for child support and am low income.
Help!!!