Dependents

Why is asking if there is a custody agreement if filing as married filing separately and claiming son on my return.  I was never divorced and answered status as married in a very early question.  If answer no then says person doesn't qualify as my dependent.  Doesn't make sense.  I'm trying to recreate my 2019 return (TT changed the type of file it created and my backup software didn't capture it and I lost it when had computer issues) where he was a dependent.