You may want to read http://talk.collegeconfidential.com/financial-aid-scholarships/2083835-faq-divorced-parents-financial-aid-and-net-price-calculators.html for background information on the divorced parent situation.
For FAFSA purposes, the custodial parent is the parent you lived with the greater amount of time during the past 12 months: https://studentaid.ed.gov/sa/fafsa/filling-out/parent-info#divorced-separated-living-apart
CSS Profile appears to have a similar definition: https://learn.collegeboard.org/css-profile-overview/16/