The PDF shows that it's the "supply" (for lack of a better term) to the overflow tank. While it might be possible to reattach the fitting with epoxy, jbweld, etc., my experience has been that repairs to the plastic fittings of Behr and other MB radiators is that it's a short term fix. Plastic neck failures due to vibration/stress seems to be a common fault. I wish they still sold the metal sleeved radiators again.